Compositions for inducing differentiation into retinal cells from retinal progenitor cells or inducing proliferation of retinal cells comprising wnt signaling pathway activators

ABSTRACT

Disclosed is a composition for inducing the proliferation of retinal cells or the differentiation of retinal progenitor cells into retinal cells. The composition, similar to in vivo conditions for development during embryogenesis, induces stem cells to differentiate into a multitude of photoreceptor cells at high yield within a short period of time, without an additional gene transfer. In addition, the differentiated photoreceptor cells are useful in cellular therapy because they, when transplanted into degenerated or injured retinas, can be engrafted and fused within the retinas to prevent or cure retinal degeneration.

TECHNICAL FIELD

The present invention relates to compositions for inducing the proliferation of retinal cells and the differentiation of retinal progenitor cells into retinal cells. More particularly, the present invention relates to compositions for inducing the differentiation of retinal progenitor cells into retinal cells, especially photoreceptor cells and the proliferation of photoreceptor cells that comprise a Wnt signaling pathway activator.

BACKGROUND ART

Blindness is the medical condition of lacking visual perception for physiological or neurological reasons. As many as tens of millions of people, which accounts for 0.2-0.5% of the population of the world, are affected with blindness, and are suffering from great losses in personal, social and economical respects. Retinal photoreceptor degeneration is one of the more dominant etiologies of blindness, caused innately or by other various factors, including retinal dysplasia, retinal degeneration, aged macular degeneration, diabetic retinopathy, retinitis pigmentosa, congenital retinal dystrophy, Leber congenital amaurosis, retinal detachment, glaucoma, optic neuropathy, and trauma. No drugs have been developed for the fundamental treatment of these diseases thus far. To date, the replacement of dysfunctional photoreceptor cells, the alpha and omega of these retinal diseases, with new ones is regarded as the only promising therapy. Photoreceptor cell implantation is thought to prevent blindness or recover imperfect eyesight by delaying or restraining retinal degeneration, regenerating degenerated retina, and enhancing retinal functions.

Stem cells have become a candidate useful for cell therapy of retinal diseases including bone marrow stem cells (BMSC), cord blood stem cells, amniotic fluid stem cells, fat stem cells, retinal stem cells (RSC), embryonic stem cells (ESC), induced pluripotent stem cells (iPSC) and somatic cell nuclear transfer cells (SCNT).

No significant research results have been yet suggested regarding the differentiation of stem cells into retinal cells (particularly, photoreceptor cells) and cell therapy based thereon. The differentiation of these stem cells into retinal cells might make it possible 1) to guarantee an infinite cell source for efficient cell therapy, 2) to identify the differentiation mechanism from embryonic cells and retinal progenitors into retinal cells, which has remained unclear, 3) to find retina differentiation-related genes and molecules and lesions thereby, 4) to understand the pathogenesis of retinal degenerative diseases, and 5) to develop drugs for preventing retinal degeneration and protecting the retina.

Since the first establishment thereof, human embryonic stem cell lines have been suggested to have the ability to differentiate into various types of cells which are useful for the cellular therapy of various diseases. Human embryonic stem cells appear to have a high potential when it comes to allowing the accurate examination of pathogenetic mechanisms and supplying fresh cells that can substitute for dysfunctional cells in clinical treatment. The production of human ESC derived-retinal photoreceptor cells under a completely identified reproducible condition and the use thereof in transplantation would guarantee a highly potential and effective therapy for retinal photoreceptor cell-related diseases. It has been assumed that human ESC derived-cells will have the same properties and functions as did the cells formed that were formed through a normal differentiation processes. Based on this assumption, differentiation has been induced under circumstances similar to those of the developmental stages to produce pancreatic hormone-expressing endocrine cells (D'Amour, et al., Nat. Biotechnol., 2006; 24: 1392-401), neurons (Pankratz, et al., Stem Cells 2007; 25: 1511-20), muscle cells (Barberi et al., Nat. Med., 2007; 13: 642-8), and vascular endothelial cells (Wang, et al., Nat. Biotechnol., 2007; 25: 317-8). Also, many attempts have been made to differentiate human ESC into photoreceptor cells which may be effectively used to treat retinal diseases, but this ended with failure for most cases.

In fact, differentiation into retinal progenitor cells from human embryonic stem cells is the greatest achievement made thus far in this field, but the differentiation of retinal progenitor cells into photoreceptor cells failed (differentiation rate of less than 0.01%) (Lamba et al., Proc. Natl. Acad. Sci. USA, 2006; 103: 12769-74). One report held that human embryonic stem cells were successfully induced to differentiate into photoreceptor cells, but the method used therein requires more than 200 days in total for the differentiation, with a differentiation rate of as low as 8%, and thus is impossible to apply to the clinical treatment of blindness (Osakada et al., Nat. Biotechnol., 2008; 26: 215-24).

The Wnt signaling pathway participates in regulating various processes during embryogenesis, including tissue development, cell proliferation, morphology, motility and cell-fate determination, etc (Wodarz & Nusse, Annu. Rev. Cell Dev. Biol., 1998; 14: 59-88). It is known to promote or regulate differentiation depending on tissue type and differentiation level. To date, in the context of embryonic development and cell biology, the Wnt signaling pathway including Wnt3a has been reported to be deeply involved in the regulation of cellular differentiation and the maintenance and proliferation of undifferentiated cells (Aubert, et al., Nat. Biotechnol., 2002; 20: 1240-5). Nowhere has the effect of the Wnt signaling pathway on cell differentiation and maturation associated with vertebrate eye patterning and neurogenesis been reported in the prior art.

DISCLOSURE Technical Problem

Leading to the present invention, intensive and thorough research into the differentiation of human ESC into retinal cells, particularly photoreceptor cells, conducted by the present inventors, using chemically defined, resulted in the finding that when in vitro conditions for differentiation into photoreceptor cells, similar to in vivo conditions, in which differentiation-associated factors and their inhibitors were employed, was applied, the Wnt signaling pathway activators played an important role in the proliferation of retinal cells and the differentiation and maturation from stem cells into retinal cells, particularly photoreceptor cells.

Technical Solution

It is therefore an object of the present invention to provide a composition for inducing the proliferation of retinal cells, comprising a Wnt signaling pathway activator. It is another object of the present invention to provide a composition for inducing differentiation from retinal progenitor cells into retinal cells, particularly photoreceptor cells, comprising a Wnt signaling pathway activator.

Advantageous Effects

As described above, the composition of the present invention allows stem cells to differentiate into a multitude of photoreceptor cells at high yield within a short period of time, without an additional gene transfer. Able to generating a multitude of photoreceptor cells at high yield, the composition of the present invention does neither require the isolation of photoreceptor cells through flow cytometry nor an additional photoreceptor proliferation so that the differentiated photoreceptor cells may be readily transplanted into degenerated or injured retinas. In addition, the differentiated photoreceptor cells are useful in cellular therapy because they, when transplanted into degenerated or injured retinas, can be engrafted and fused within the retinas to prevent or cure retinal degeneration.

DESCRIPTION OF DRAWINGS

FIG. 1 is of cytomorphological microphotographs:

(A) Left. Typical cell floc of hESCs in an undifferentiated state (29 passages; 40× magnification), after being cultured for 5 days from cells of passage number 28. Characterized by a definite separation from adjacent MEF feeder cells. Having plain surface and uniform morphology.

(A) Right. Floating aggregates (40× magnification), being cultured for 4 days in ultra-low attachment plates after isolation from the hESC floc of FIG. 1A Left. Spherical morphology. Consisting of approx. 292 53 cells per floating aggregate.

(B)-(D). Morphological microphotographs of cells differentiating into retinal cells.

(B). Cells on Day 14 after induction of the differentiation, that is, after the floating aggregates were transferred to poly-D-lysine/laminin-coated plates and cultured for 10 days therein, which was on Day 14 after the induction of the differentiation of the undifferentiated hESCs. The cells were observed to be separated from the floating aggregates and to undergo differentiation. Morphology of cells in the early stage of differentiation, with meager cytoplasm and round, large nuclei.

(C). Cells on Day 19 after the induction of the differentiation, that is, cells after the undifferentiated hESCs were induced to differentiate for 19 days. They differentiated into retinal progenitor cells, with concomitant active proliferation. Cell flocs under active proliferation and differentiation formed an eddy formation or a rosette configuration.

(D). Cells on Day 21 after induction of the differentiation, showing an increased number of cells resulting from active proliferation. The cells became richer in cytoplasm and the size of their nucleus was smaller than those of FIG. 1C as the differentiation progressed. The cells appeared to function in response to light.

(E)-(H). Various morphologies of cell flocs on Day 29 after induction of the differentiation.

(E). The morphology of most cells, particularly observed in densely populated regions. With the progress of differentiation, the cells showed the same cellularity, but had a richer cytoplasm and smaller and denser nuclei, compared to those on Day 21 after induction.

(F). The morphology of cells in a scarcely populated region. The cell flocs showed directivity and moved towards a certain point which depended on the cell cluster. More plentiful, opposite end-pointed cytoplasms and spindle-like nuclei were observed.

(G). Cell flocs, some having a multiple of nerve bundles.

(H) Left. Cell flocs some of which show long neural axons.

(H) Right. Cell flocs some of which show the morphology of differentiated neurons.

-   -   Microscopic field: (A) (left, right: 40× magnification); (B)-(G)         (left: 50× magnification; right: 200× magnification); (H) (left         and right: 200× magnification).

FIG. 2 is a graph showing changes in expression levels of the retinal cell markers Crx, recoverin, rhodopsin, peripherin2 and Ki67 with culture time period.

FIG. 3 is a set of microphotographs showing the cells obtained by differentiation into retinal cells for 29 days, which were immunostained for recoverin and rhodopsin, both indicative of photoreceptor cells.

After hESCs were induced to differentiate into photoreceptor cells, an examination was made of the expression of photoreceptor cell-specific proteins. More than 80% of the differentiated cells tested positive to both recoverin (a universal photoreceptor cell marker) and rhodopsin (characteristic of rod photoreceptor cells).

(A) and (B). Flocs of differentiated photoreceptor cells.

(C). Individual cells in scarcely populated regions.

Recoverin and rhodopsin are distinctively expressed in the differentiated photoreceptor cells.

-   -   Microscopic field: (A) 100× magnification; (B) 200×         magnification; (C) 400× magnification.     -   Merge: superimposed photographs of cells which were fluorescent         immunostained for recoverin and rhodopsin, cells expressing both         the antigens being represented yellow (green+red).     -   ⁺Merge/DAPI: DAPI is a nucleus-stained cell population. The         Merge/DAPI images are of superimposed fluorescence photographs         to detect the expression of both recoverin and rhodopsin and the         expression of DAPI, showing cell contours and the expression         pattern of the two antigens, simultaneously.

FIG. 4 is of fluorescence microphotographs of the cells obtained after inducing differentiation into retinal cells for 29 days, showing the expression of the photoreceptor cell markers rhodopsin, rom-1 and peripherin2.

The differentiated photoreceptor cells were observed to express Rom-1 and peripherin2, both characteristic of the outer segment of rhodopsin-positive rod photoreceptor cells.

(A). Cell flocs positive to both rhodopsin and rom-1.

(B). Individual cells positive to both rhodopsin and rom-1. Within each cell, rhodopsin and rom-1 were expressed at distinctly different positions. With the progress of differentiation, rhodopsin was expressed in the inner cytoplasm while rom-1 was positioned at the outermost cytoplasm.

(C). Flocs of the differentiated photoreceptor cells which were positive to both rhodopsin and peripherin2.

(D). Individual cells positive to both rhodopsin and peripherin2.

-   -   Microscopic field: (A) 100× magnification; (B) 400×         magnification; (C) 100× magnification; (D) 400× magnification.

FIG. 5 is of fluorescence microphotographs of the cells obtained after inducing differentiation into retinal cells for 29 days, showing the expression of the photoreceptor cell markers rhodopsin, phosducin and Pde6b. These proteins are responsible for the response to light, demonstrating that the differentiated photoreceptor cells are exhibiting their proper functions.

(A). Flocs of the differentiated photoreceptor cells which are positive to both rhodopsin and phosducin.

(B). Individual cells positive to both rhodopsin and phosducin.

(C). Flocs of the differentiated photoreceptor cells positive to both rhodopsin and Pde6b.

(D). Individual cells positive to rhodopsin and Pde6b.

-   -   Microscopic field: (A) 100× magnification; (B) 400×         magnification; (C) 100× magnification; (D) 400× magnification.

FIG. 6 is of fluorescence microphotographs of cells obtained after inducing differentiation into retinal cells for 29 days, showing the expression of the photoreceptor cell markers rhodopsin and synaptophysin.

(A). Flocs of the differentiated photoreceptor cells positive to both rhodopsin and synaptophysin. The expression of these proteins demonstrates that the differentiated photoreceptor cells are in synapse interaction with other retinal neurons and are participating in the formation of retinal nerve circuits.

(B). Individual cells positive to rhodopsin and synaptophysin.

-   -   Microscopic fields: (A) 100× magnification; (B) 400×         magnification.

FIG. 7 is of fluorescence microphotographs of cells which were obtained after inducing differentiation into retinal cells for 29 days and immunostained against cone photoreceptor cells.

Left panel. Cell flocs positive to blue opsin.

Right panel. Individual cells positive to blue opsin.

The expression of blue-opsin is evidence that the differentiated cells are blue opsin-cone photoreceptor cells.

-   -   Microscopic field (left) 100× magnification; (right) 400×         magnification.

FIG. 8 is of fluorescence microphotographs of cells which were obtained after inducing differentiation into retinal cells for 29 days and which were immunostained against characteristic photoreceptor cells. Various types of cells which had undergone further differentiation were observed.

Left. Cells positive to both recoverin and rhodopsin, showing a morphology characteristic of photoreceptor cells.

Middle. Cells positive to synaptophysin and rhodopsin, showing further mature differentiation.

Right. Rhodopsin-positive cells characterized by rich rhodopsin molecules within the cytoplasm.

-   -   Microscopic field 400× magnification.

FIG. 9 is of fluorescence microphotographs of cells which were obtained after inducing differentiation into retinal cells for 29 days and which were immunostained against neural retinal progenitor cells and photoreceptor cell precursors.

(A). Cell flocs positive to both Rax and Pax6.

(B). Individual cells positive to both Rax and Pax6.

Most cells were observed to express both the antigens although the expression level was different between them, indicating that the retinal cells obtained after differentiation induction for 29 days were derived from neural retinal progenitor cells.

(C). Cell flocs positive to the proliferative cell marker Ki67 and the photoreceptor cell precursor marker Crx.

(D). Individual cells positive to both Ki67 and Crx.

Most Crx-positive cells do not express Ki67. This coincides with the fact that photoreceptor cell precursors express Crx immediately after leaving the cell proliferation cycle. Sometimes, a minority of Crx-positive cells still continue to express Ki67.

-   -   Microscopic field (A) 100× magnification; (B) 400×         magnification; (C) 100× magnification; (D) 400× magnification.

FIG. 10 is of fluorescence microphotographs of cells which were obtained after inducing differentiation into retinal cells for 29 days and immunostained against retinal cells other than photoreceptor cells.

(A). Cell flocs (left) and individual cells, positive to both Islet-1 and NF-200, which gives evidence of retinal ganglion cells because nuclei and axons are positive to Islet-1 and NF-200, respectively.

(B). Cell flocs (left) and individual cells (right) positive to PKC-α, which gives evidence of bipolar cells.

(C). Cell flocs (left) and individual cells (right) positive to Prox-1, which gives evidence of horizontal cells.

(D). Cell flocs (left) and individual cells (right) positive to GFAP, which gives evidence of Muller glial cells.

(E). Cell flocs (left) and individual cells (right) positive to both Rpe65 and ZO-1, which gives evidence of retinal pigmented epithelium.

-   -   Microscopic field: (A). Left. 100× magnification, Right. 400×         magnification; (B) Left. 100× magnification, Right. 400×         magnification; (C) Left. 100× magnification, Right. 400×         magnification; (D) Left. 100× magnification, Right. 400×         magnification; (E) Left. 100× magnification, Right. 400×         magnification.

FIG. 11 is of fluorescence microphotographs of the cells resulting from inducing differentiation into retinal cells for 29 days, with BIO and purmorphamine used respectively instead of Wnt3a and Shh, which were immunostained against photoreceptor cell precursors and photoreceptor cells.

(A). Cell flocs positive to both the proliferative cell marker Ki67 and the photoreceptor cell precursor-specific antigen Crx.

(B). Individual cells positive to both Ki67 and Crx.

(C). Cell flocs positive to the photoreceptor cell markers recoverin and rhodopsin.

(D). Individual cells positive to recoverin and rhodopsin.

-   -   Microscopic field (A) 100× magnification; (B) 400×         magnification; (C) 100× magnification; (D) 400× magnification.

FIG. 12 is of fluorescence microphotographs of the cells resulting from inducing differentiation into retinal cells for 29 days, with BIO and purmorphamine used respectively instead of Want3a and Shh, which were immunostained for the photoreceptor cell markers rhodopsin, peripherin2 and rom-1.

(A). Cell flocs positive to both rhodopsin and peripherin2.

(B). Individual cells positive to both rhodopsin and peripherin2.

(C). Cell flocs positive to both rhodopsin and rom-1.

(D). Individual cells positive to both rhodopsin and rom-1.

-   -   Microscopic field: (A) 100× magnification; (B) 400×         magnification; (C) 100× magnification; (D) 400× magnification.

FIG. 13 is of fluorescence microphotographs of the cells resulting from inducing differentiation into retinal cells for 29 days, with BIO and purmorphamine used respectively instead of Want3a and Shh, which were immunostained against cone photoreceptor cells.

Left. Blue-opsin-positive cell flocs.

Right. Blue-opsin-positive individual cells.

-   -   Microscopic field (Left) 100× magnification; (Right) 400×         magnification.

FIG. 14 is of microphotographs of human iPSCs.

(A). Cytomorphological microphotographs. (A) Left. Typical cell floc of human iPSCs in an undifferentiated state (passages 43; Microscopic field: 40 magnifications), after being cultured for 6 days from cells of passages 43. Characterized by definite separation from adjacent MEF feeder cells. Having a plain surface and uniform morphology, which is also characteristic of undifferentiated hESCs. (A) Right. Floating aggregates (Microscopic field: 40× magnification), being cultured for 4 days in ultra-low attachment plates after isolation from the human iPSC floc of FIG. 14A Left.

(B). Fluorescence microphotographs of undifferentiated human iPSCs immunostained for characteristic markers. Cell flocs in which most cells are positive to both SSEA4 and Nanog, which gives evidence of the continuance of undifferentiated states.

-   -   Microscopic field (leftmost) 40× magnification; (the others)         100× magnification.

FIG. 15 is a set of fluorescence microphotographs showing the cells obtained by inducing human iPSC to differentiate into retinal cells for 29 days, which were immunostained for recoverin and rhodopsin, both characteristic of photoreceptor cells. The photoreceptor cells differentiated from human iPSCs were assayed for the expression of photoreceptor cell-specific proteins.

(A). Flocs of differentiated photoreceptor cells.

(B). Individual cells at the low cell density area.

(C). Individual cells in scarcely populated regions.

Recoverin and rhodopsin are distinctively expressed in the differentiated photoreceptor cells.

-   -   Microscopic field: (A) 100× magnification; (B) 400×         magnification; (C) 400× magnification.

FIG. 16 is of photographs showing RT-PCR for genes specific for retinal cells. The cells generated by inducing undifferentiated hESCs to differentiate into retinal cells for 29 days were assayed for the mRNA expression levels of genes associated with retinal progenitor cells, photoreceptor cells and other retinal cells using RT-PCR.

(A). RT-PCR products of the retinal progenitor cell-specific genes RAX (495 bp), PAX6 (275 bp), SIX3 (307 bp), SIX6 (272 bp), LHX2 (285 bp) and CHX10 (281 bp). Inter alia, RAX and PAX6 were expressed to the same high extent as was the quantitative control gene GAPDH (PCR product size: 302 bp). In contrast, none of the developing cerebral cortex-relevant gene ARX (462 bp), the developing mesodermal gene T (541 bp), or the developing endodermal gene AFP (318 bp) were found in the RT-PCR products, suggesting that the method of the present invention is specifically directed to the mRNA expression of retina-relevant genes.

(B). RT-PCR products of genes associated with photoreceptor cells and other retinal cells. The photoreceptor cell-relevant genes CRX (353 bp), NRL (206 bp), RCVRN (150 bp), RHO (258 bp), PDE6B (409 bp), SAG (400 bp) and OPN1SW (206 bp) were observed to be amplified by RT-PCR. The retinal ganglion cell genes ATHO7 (246 bp) and POU4F2 (175 bp), the amacrine cell gene NEUROD1 (523 bp), and the bipolar cell gene ASCL1 (467 bp) were also found in the RT-PCR products.

Characteristics of photoreceptor cell-relevant genes are as follows: CRX and NRL are transcription genes characteristic of photoreceptor cell precursors and rod photoreceptor cells, respectively. RCVRN (recoverin) is a universal photoreceptor cell gene that tests positive for both cone and rod photoreceptor cells. RHO (rhodopsin) is rod photoreceptor cell specific. PDE6B and SAG (human arrestin) are involved in the phototransduction of photoreceptor cells. The expression of these genes gives evidence of the development and maturation of the photoreceptor cell's own functions. OPN1SW is characteristic of short wave (blue opsin)-cone photoreceptor cells. M: marker.

FIG. 17 shows RT-PCR and base sequencing results of genes characteristic of photoreceptor cells.

RT-PCR was performed on the cells generated by inducing undifferentiated hESCs to differentiate into retinal cells for 29 days, so that the photoreceptor cell-specific genes RCVRN (NM_(—)002903.2) and RHO (NM_(—)000539.3) could be detected. The RT-PCR products were identified as RCVRN and RHO by base sequencing.

(A). Agarose gel electrophoresis of the RT-PCR products showing RCVRN at 150 bp and RHO at 258 bp. M: marker.

(B). Chromatogram of base sequencing analysis, showing an RCVRN base sequence (top) and an RHO base sequence (bottom). The RCVRN and RHO gene base sequences were found to perfectly coincide with human standard sequences (http://www.ncbi.nlm.nih.gov/), indicating that the photoreceptor cells express human RCVRN and RHO genes.

FIG. 18 is of electroretinograms of the retinal degeneration mouse rd/SCID which had been or had not been transplanted with the hESC-derived photoreceptor cells.

(A). Electroretinograms of 8-week-old, non-transplanted mice. No characteristic ERG wave forms were found. The ERG b-wave had an amplitude of 6.29 μV for the right eye and 0.0542 μV for the left eye.

(B). Electroretinograms of 8-week-old mice 4 weeks after the transplantation. Compared to the non-transplanted right eye, the EGR b-wave from the photoreceptor cell-transplanted left eye formed characteristic wave forms, with an amplitude of as high as 74.5 μV. The rd/SCID mice transplanted with the hESC-derived photoreceptor cells showed definite responses to light stimuli as measured by electroretinography.

FIG. 19 is a graph comparing the amplitude of the b-wave between rd/SCID mice with retinal degeneration which had been transplanted or had not been transplanted with the hESC-derived photoreceptor cells.

The ERG b-wave from the photoreceptor cell-transplanted rd/SCID mice formed characteristic wave forms, with an amplitude of 48.4(±3.4) μV (sample size=13). In contrast, characteristic wave forms where nowhere to be found in the ERG of the non-transplanted group, which showed a b-wave amplitude of 10.3(±2.5) μV (sample size=17) which is different from that of the transplanted group with statistical significance (p<0.0001) (Table 6, FIG. 19).

FIG. 20 is of fluorescence microphotographs after the hESC-derived photoreceptor cells had been transplanted into the mouse model of retinal degeneration (rd/SCID).

Four weeks after the transplantation, the hESC-derived photoreceptor cells were analyzed for engraftment into the retina using rhodopsin and recoverin, both characteristic of human mitochondria and photoreceptor cells. When cells positive to rhodopsin and recoverin showed a positive response to a human mitochondrial antigen, they were decided to be hESC-derived photoreceptor cells.

(A). Immunostained human-specific mitochondria and rhodopsin in the transplanted group. A new outer nuclear layer (ONL) was formed of a 4- or 5-fold, rhodopsin-positive photoreceptor cell layer.

(B). Immunostained human-specific mitochondria and rhodopsin in the non-transplanted group of rd/SCID mice of the same age (8 weeks old) which served as a control. Only a single outer nuclear layer was observed, consisting mostly of cone photoreceptor cells. Almost no rod photoreceptor cells were observed due to degeneration while only two residual cells which were undergoing degeneration were detected.

(C). Immunostained human-specific mitochondria and recoverin in the transplanted group. A 4- or 5-fold recoverin-positive cell layer formed a new outer nuclear layer. In the transplanted group, a 4- or 5-fold recoverin-positive cell layer was formed in the outer nuclear layer as well as in the inner nuclear layer (INL).

(D). Immunostained human-specific mitochondria and recoverin in the non-transplanted group used as a control. Positive responses were detected in a total of 40 cells. A single recoverin-positive outer nuclear layer consisted of cone-photoreceptor cells while the recoverin-positive inner nuclear layer was formed of cone-bipolar cells.

-   -   Microscopic field: (A) and (C) Left. 200× magnification;         (A)-(D): 400× magnification.     -   ONL: outer nuclear layer

INL: inner nuclear layer

RGC: retinal ganglion cell

FIG. 21 is a graph showing engraftment results after human ESC-derived photoreceptor cells were transplanted into the mouse model of retinal degeneration (rd/SCID).

In the non-transplanted group, rhodopsin was detected in only two of a total of 199 cells per observation microscopic field (positive rate: 1.0%). On the other hand, 88 of a total of 215 cells per microscopic field were rhodopsin positive in the transplanted group (positive rate: 40.8%) (p<0.0001). Accordingly, the transplanted rod photoreceptor cells were found to occupy approximately 40% of the total area of the retinal sections. Positive responses to recoverin were detected in 40 of the total of 168 cells per microscopic field in the non-transplanted group (positive rate: 23.8%), but in 120 of the total 292 cells per microscopic field in the transplanted group (positive rate: 41.0%), with statistical significance (p<0.0001).

FIG. 22 is of fluorescence microphotographs after the hESC-derived photoreceptor cells had been transplanted into the mouse model of retinal degeneration (rd/SCID).

Four weeks after the transplantation, human mitochondria and the photoreceptor cell antigen synaptophysin were immunostained and analyzed. In the non-transplanted group, recoverin means bipolar cells in the inner nuclear layer and cone photoreceptor cells in the outer nuclear layer. In the transplanted group, a 4- or 5-fold synaptophysin-positive cell layer was found to form a new outer nuclear layer, suggesting that the photoreceptor cells in the newly formed outer nuclear layer are in synaptic interaction with other intraretinal cells within the retinas of the transplanted mice.

-   -   Microscopic field: Left. 200× magnification; the others 400×         magnification.

FIG. 23 is a graph showing mean cell counts according to combinations of the differentiation factors Wnt3a (W), Shh (S) and retinoic acid (R).

FIG. 24 is a graph showing positive rates of photoreceptor cell markers versus the concentrations of Wnt3a and its inhibitor Dkk-1, in the absence of Shh and retinoic acid (*: P<0.0001; **: P=0.0381)

BEST MODE

In accordance with an aspect thereof, the present invention pertains to a composition for promoting the proliferation of retinal cells, comprising a Wnt signaling pathway activator.

The term “Wnt signaling pathway activator”, as used herein, is intended to refer to a substance activating the Wnt signaling pathway which has been found to regulate various processes during embryogenesis, including cell-fate determination, reconstruction of organization, polarity, morphology, adhesion and growth, and the maintenance and proliferation of undifferentiated cells (Logan & Nusse, Annu Rev Cell Dev Biol. 2004; 20: 781-810). As long as it transduces Wnt-mediated or beta-catenin-mediated signals, any activator may be included within the Wnt signaling pathway. The Wnt signaling pathway is a series of processes that are initiated by the binding of the trigger Wnt to its receptor or mediated by the stabilization of the downstream factor β-catenin. The following is a description of how to activate Wnt signaling pathway.

1) By adding a Wnt protein: Wnt, a first trigger of the Wnt signaling pathway, is a family of secreted glycoproteins. 19 Wnts have been identified: Wnt1, Wnt2, Wnt2b, Wnt3, Wnt3a, Wnt4, Wnt5a, Wnt5b, Wnt6, Wnt7a, Wnt7b, Wnt8a, Wnt8b, Wnt9a, Wnt9b, Wnt10a, Wnt10b, Wnt11, and Wnt16b.

2) By increasing the level of β-catenin: most cells respond to Wnt signaling pathway by an increase in the level of β-catenin. That is, an increase in dephosphorylated β-catenin level or the stabilization of β-catenin means the translocation of β-catenin into the nucleus.

3) By phosphorylation of dishevelled or phosphorylation of a Wnt-accosiated receptor, LRP tail.

4) By using GSK3 (glycogen synthase kinase 3) inhibitors: lithium (Li), LiCl, bivalent Zn, BIO (6-bromoindirubin-3′-oxime), SB216763, SB415286, QS11 hydrate, TWS119, Kenpaullone, alsterpaullone, indirubin-3′-oxime, TDZD-8, and Ro 31-8220 methanesulfonate salt.

5) By blocking negative regulators of Wnt signaling pathway, such as Axin and APC, or by using RNAi.

6) With activators of the Wnt pathway, such as norrin and R-spondin2: Norrin binds to Frizzled4 receptor while R-spondin2 interacts with Frizzled8 and LRP6.

7) By gene transfer, including transfection: one can activate Wnt signaling pathway using either Wnt overexpression constructs or β-catenin overexpression constructs.

In a preferred embodiment, the Wnt signaling pathway activators may be employed without limitation. Preferred are Wnt1, Wnt2, Wnt2b, Wnt3, Wnt3a, Wnt4, Wnt5a, Wnt5b, Wnt6, Wnt7a, Wnt7b, Wnt8a, Wnt8b, Wnt9a, Wnt9b, Wnt10a, Wnt10b, Wnt11, Wnt16b; substances increasing β-catenin levels; GSK3 inhibitors such as lithium, LiCl, bivalent zinc, BIO, SB216763, SB415286, CHIR99021, QS11 hydrate, TWS119, Kenpaullone, alsterpaullone, indirubin-3′-oxime, TDZD-8 and Ro 31-8220 methanesulfonate salt; Axin inibiotors, APC inhibitors, norrin and R-spondin 2, and the most preferred are Wnt3a, Wnt1, Wnt5a, Wnt11, norrin, LiCl, BIO and SB415286.

In a preferred embodiment of the present invention, the composition for inducing the proliferation of retinal cells contains the Wnt signaling pathway activator except for LiCl, BIO and SB415286 in an amount of from 0.01 to 500 ng/ml, preferably in an amount of from 0.1 to 200 ng/ml, and more preferably in an amount of from 1 to 100 ng/ml. Among the Wnt signaling pathway activators, LiCl is used in the medium in an amount of 0.1 to 50 mM, preferably in an amount of 0.5 to 10 mM, and more preferably in an amount of 1 to 10 mM; BIO is used in an amount of 0.1 to 50 μM, preferably in an amount of 0.1 to 10 μM, and more preferably in an amount of 0.5 to 5 μM; SB415286 is used in an amount of 0.1 to 500 μM, preferably in an amount of 1 to 100 μM, and more preferably in an amount of 5 to 50 μM. In a modification of the embodiment, the medium may contain 50 ng/ml of Wnt3a or Wnt1; 50 or 100 ng/ml of Wnt5a and Wnt11; 50 ng/ml of norrin; 2.5 or 5 mM of LiCl; 2 μM of BIO, or 30 μM of SB415286. The term “retina”, as used herein, refers to light-sensitive tissue. The retina is the innermost (sensory) transparent coat in the eyeball and is directly relevant to vision. Just outside the neurosensory retina is the retinal pigment epithelium consisting of pigmented cells. In a broader sense, the retina includes the inner sensory coat and the outer retinal pigmented epithelium. The retina is located at the back of the eye and originates as outgrowths of the developing brain in embryonic development. The retina is like a five-layered cake, consisting of three nuclear layers with two network layers intercalated therebetween. The three nuclear layers are: the outermost nuclear layer consisting of photoreceptor cells; inner nuclear layer consisting of horizontal cells, bipolar cells, amacrine cells, and Muller glias; and the innermost retinal ganglion layer consisting of the nuclei of retinal ganglion cells. After passing through the cornea and the lens of the eye, light reaches the outer nuclear layer through the retinal ganglion layer and the inner layer in that order, producing neural impulses at photoreceptor cells. These neural impulses are transduced in a reverse direction. That is, when photoreceptor cells are stimulated by the neural impulses, nerve currents are transmitted to the inner nuclear layer and then into optic nerve fibers through the retinal ganglion cell layer.

As used herein, the term “retinal cell” is intended to refer to a cell constituting a part of the retina, including photoreceptor cells such as rod and cone photoreceptor cells, retinal ganglion cell, horizontal cells, bipolar cells, amacrine cells, Muller glial cells, and retinal pigmented epithelium. The composition of the present invention induces retinal cells, particularly photoreceptor cells to proliferate.

As used herein, the term “photoreceptor cell” refers to a specialized type of neuron found in the eye's retina that is capable of phototransduction and allowing shapes and colors to be recognized: when light reaches the retina through the cornea and the lens, the photoreceptor cell converts the light energy into electric energy which is then transmitted into the brain. There are two main types of photoreceptor cells: rods and cones, which are adapted for low light and bright light, respectively. Cone cells gradually become denser towards the center of the retina, that is, the yellow spot, and function to perceive images and colors while rod cells are distributed predominantly at the periphery of the retina, allowing the perception of images and light. Photoreceptor cells are characterized by the ability to express at least one, two or three markers selected from among recoverin (rod photoreceptor cells, cone photoreceptor cells), rhodopsin (rod photoreceptor cells), peripherin2 (rod photoreceptor cells), rom1 (rod photoreceptor cells, cone photoreceptor cells), Pde6b (rod photoreceptor cells), arrestin sag (rod photoreceptor cells), phosducin (rod photoreceptor cells, cone photoreceptor cells), synaptophysin (rod photoreceptor cells, cone photoreceptor cells), red/green opsin (cone photoreceptor cells), and blue opsin (cone photoreceptor cells).

In a preferred embodiment, the retinal cells or photoreceptor cells of the present invention may be derived from animals including humans. As used herein, the term “animal” is intended to include humans, primates, cows, pigs, sheep, horses, dogs, mice, rats, and cats, humans being preferred.

In an embodiment of the present invention, a Wnt signaling pathway activator was identified as playing an important role in the proliferation of retinal cells. In this regard, the Wnt signaling pathway activator Wnt3a (W), the Shh signaling pathway activator Shh (S), and retinoic acid (R) were combined with one another to afford various compositions which were then applied to hESCs at different time points, followed by culturing the cells for 29 days. After completion of the differentiation, differentiated retinal cells were counted (Example 9).

The highest cell population was measured upon the use of W+/S+/R+ (3.98(±0.64)×10⁶ cells), followed by 2.74(±0.36)×10⁶ cells in W+/S−/R+ and 2.21(±0.67)×10⁶ cells in W+/S+/R−. W−/S−/R− and W−/S+/R+ proliferated the cells at a density of as low as 0.87(±0.38)×10⁶ and 0.73(±0.16)×10⁶, respectively (FIG. 23 and Table 8). The data indicate that cell proliferation is independent of the presence of S and R, but depends on W. A great part of the cell proliferation results from the effect of Wnt3a. When comparing the two media W+/S+/R+ and W−/S+/R+, the cell populations (3.98×10⁶ vs. 0.73×10⁶) differ by five times (FIG. 23 and Table 8). This is another evidence that Wnt3a plays a critical role in cell proliferation. After being cultured four weeks in the (W+/S+/R+) culture for 4 weeks, the undifferentiated cells proliferated to 3.98×10⁶ differentiated cells (FIG. 23 and Table 8), which was 257-fold higher than the population of the initial cells, for the first time indicating that the Wnt signaling pathway activator can be used for proliferating retinal cells.

As used herein, the term “stem cell” refers to a cell with pluripotency to give rise to all derivatives of the three primary germ layers (the endoderm, mesoderm and ectoderm) or one with multipotency to differentiate into mature cells closely related in tissue type and function.

As used herein, the term “animal” is intended to include humans, primates, cows, pigs, sheep, horses, dogs, mice, rats, and cats, humans being preferred.

As used herein, the term “embryonic stem cell” refers to a pluripotent cell derived from the inner cell mass of the blastocyst immediately before the nidation of a fertilized egg on the uterine wall, which can differentiate into any type of animal cells, and is intended in a broader sense to include stem cell-like cells such as embryoid bodies and induced pluripotent stem (iPS) cells.

The term “adult stem cell”, as used herein, is intended to refer to a multipotent cell which is isolated from tissues and cultured ex vivo, and to include bone marrow stem cells, cord blood stem cells, amniotic fluid stem cells, fat stem cells, retinal stem cells, intraretinal Muller glial cells and neural stem cells.

In accordance with another aspect, the present invention pertains to a composition for inducing retinal progenitor cells to differentiate into retinal cells, comprising a Wnt singling pathway activator. Preferably, the retinal cells are photoreceptor cells.

The term “retinal progenitor cell”, as used herein, is intended to refer to a multipotent progenitor cell which can differentiate into cells present in the retina and retinal pigmented epithelial cells. Retinal progenitor cells may be cells which have been differentiated from various stem cells, that is, retinal stem cells, cord blood stem cells, amniotic fluid stem cells, fat stem cells, bone marrow stem cells, adipose stem cells, neural stem cells, embryonic stem cells, induced pluripotent stem cells, or somatic cells somatic cell nuclear transfer cells or may be present within or isolated from the retina. A retinal progenitor cell can, in general, undergo symmetric or asymmetric division and thus can either differentiate into various types of retinal cells or retinal pigmented epithelial cells, or produce two further retinal progenitor cells. Thus, it should be understood that the cells that are used in the culturing step to differentiate into retinal progenitor cells include various types of cells which were generated during differentiation from stem cells into retinal cells, as well as retinal progenitor cells. Retinal progenitor cells include neural retinal progenitor cells and retinal pigmented epithelial progenitor cells and are characterized by at least one, two or three markers selected from among Rax, Pax6, Chx10, Otx2, Sox2, Lhx2, Six3, Six6, and Mitf.

As used therein, the term “progenitor cell” or “precursor” refers to a cell capable of asymmetric division. asymmetric division refers to situations in which a progenitor cell or a precursor can, with a certain probability, either produce two further progenitor or precursor cells or differentiate, so that although they have undergone the same rounds of passages, the resulting cells may have different ages and properties.

In connection with retinal development, as mentioned above, retinal progenitor cells are able to differentiate into various types of intraretinal cells (rod and cone photoreceptor cells, retinal ganglion cell, horizontal cells, bipolar cells, amacrine cells, Muller glial cells, etc.) and retinal pigmented epithelium, featuring positive expression of markers such as Crx, recoverin, rhodopsin, red/green opsin, blue opsin, peripherin2, PDE6B, SAG, Islet1/NF200, Prox1, PKC-a, Hu C/D, GFAP, and RPE65. However, the expression level and positive ratio of these markers become weaker in retinal progenitor cells than in mature retinal cells or retinal pigmented epithelium.

As used herein, the term “neural retinal progenitor cells” is intended to mean the retinal progenitor cells which favor neurons. That is, neural retinal progenitor cells are herein progenitor cells determined to differentiate into intraretinal neurons (rod and cone photoreceptor cells, retinal ganglion cells, horizontal cells, bipolar cells, amacrine cells, and Muller glial cells). A neural retinal progenitor cell can, in general, undergo symmetric or asymmetric division, either differentiating into various types of retinal cells or retinal pigmented epithelial cells, or producing two further retinal progenitor cells. Thus, it should be understood that the cells in the step of culturing that differentiate into neural retinal progenitor cells include various types of cells generated during the differentiation of stem cells into retinal cells as well as neural retinal progenitor cells. Neural retinal progenitor cells are characterized by expressing at least one, two or three markers selected from among Rax, Pax6, Chx10 and Crx.

In addition to expressing these markers, neural retinal progenitor cells may be characterized by the ability to express Crx, recoverin and rhodopsin, which are the markers of cells of the next differentiation stage, that is, photoreceptor cell precursors and photoreceptor cells. On the contrary, neural retinal progenitor cells are observed to have a decreased expression level of Otx2, Sox2, Lhx2, Six3, Six6 and Mitf, which are markers characteristic of retinal progenitor cells that manifest themselves in the previous differentiation stage.

As used herein, the term “retinal pigmented epithelial progenitor cell” is intended to refer to a differentiated retinal progenitor cell which favors retinal pigmented epithelium. Retinal pigmented epithelial progenitor cells are characterized by expressing one or more markers selected from among Mift and Pax6.

In a preferred embodiment, differentiation from retinal progenitor cells into retinal cells may be achieved by (a) culturing stem cell-derived retinal progenitor cells in a composition containing a Wnt signaling pathway activator to differentiate them into neural retinal progenitor cells; (b) culturing the neural retinal progenitor cells in a composition containing a Wnt signaling pathway activator to differentiate them into photoreceptor cell precursors; and (c) culturing the photoreceptor cell precursors in a composition containing a Wnt signaling pathway activator to differentiate them into retinal cells including photoreceptor cells.

As used herein, the term “photoreceptor cell precursor” means a precursor favoring differentiation into a photoreceptor cell, characterized by one or more markers selected from among Crx (rod and cone photoreceptor cell precursors) and Nr1 (rod photoreceptor cell precursors). In addition to expressing the markers, the photoreceptor cell precursors may also be characterized by the ability to express at least one, two or three of recoverin, rhodopsin, peripherin2, and rom1, which are markers characteristic of differentiating photoreceptor cells.

So long as it activates Wnt signaling pathway, any Wnt signaling pathway activator may be used in the present invention. Examples of the Wnt signaling pathway activators useful in the present invention include Wnt1, Wnt2, Wnt2b, Wnt3, Wnt3a, Wnt4, Wnt5a, Wnt5b, Wnt6, Wnt7a, Wnt7b, Wnt8a, Wnt8b, Wnt9a, Wnt9b, Wnt10a, Wnt10b, Wnt11, Wnt16b; substances increasing β-catenin levels; GSK3 inhibitors such as lithium, LiCl, bivalent zinc, BIO, SB216763, SB415286, CHIR99021, QS11 hydrate, TWS119, Kenpaullone, alsterpaullone, indirubin-3′-oxime, TDZD-8 and Ro 31-8220 methanesulfonate salt; Axin inhibitors, APC inhibitors, norrin and R-spondin 2, with preference for Wnt3a, Wnt1, Wnt5a, Wnt11, norrin, LiCl, BIO and SB415286.

In a preferred embodiment of the present invention, the composition for inducing the proliferation of retinal cells contains the Wnt signaling pathway activator except for LiCl, BIO and SB415286 in an amount of from 0.01 to 500 ng/ml, preferably in an amount of from 0.1 to 200 ng/ml, and more preferably in an amount of from 1 to 100 ng/ml. Among the Wnt signaling pathway activators, LiCl is used in the medium in an amount of 0.1 to 50 mM, preferably in an amount of 0.5 to 10 mM, and more preferably in an amount of 1 to 10 mM; BIO is used in an amount of 0.1 to 50 μM, preferably in an amount of 0.1 to 10 μM, and more preferably in an amount of 0.5 to 5 μM; SB415286 is used in an amount of 0.1 to 500 μM, preferably in an amount of 1 to 100 μM, and more preferably in an amount of 5 to 50 μM. In a modification of the embodiment, the medium may contain 50 ng/ml of Wnt3a or Wnt1; 50 or 100 ng/ml of Wnt5a and Wnt11; 50 ng/ml of norrin; 2.5 or 5 mM of LiCl; 2 μM of BIO, or 30 μM of SB415286. In a preferred embodiment, the composition for inducing retinal progenitor cells to differentiate into neural retinal progenitor cells in step (a) may further comprise an IGF1R activator, a BMP signaling pathway inhibitor, and an FGF signaling pathway activator; the composition for inducing retinal neural progenitor cells to differentiate into photoreceptor cell precursors in step (b) may be free of the BMP signaling pathway inhibitor and the FGF signaling pathway activator and may further comprise an Shh (sonic hedgehog) signaling pathway activator; and the composition for inducing the photoreceptor cell precursors to differentiate into retinal cells including photoreceptor cells in step (c) may be the same as in the composition of step (b), with the exception that RA is further added thereto.

Any technique that is well known in the art or allows the production of retinal progenitor cells may be employed to produce retinal progenitor cells. Preferably, the retinal progenitor cells may be obtained by: (a′) culturing stem cells in a medium containing an IGF1R activator, a Wnt signaling pathway inhibitor and a BMP signaling pathway inhibitor to differentiate them into eye field precursors in the form of floating aggregates; and (b′) culturing the eye field precursors in the form of floating aggregates in the same medium, but further comprising an FGF signaling pathway activator to differentiate them into retinal progenitor cells.

In a preferred embodiment, when cultured, the floating aggregates of eye field precursors may be grown adhering to a plate. Any cell-adhering plate well known in the art may be employed. Preferably, it is coated with an extracellular matrix, such as poly-D-lysine, laminin, poly-L-lysine, matrigel, agar, polyornithine, gelatin, collagen, fibronectin or vitronectin. Most preferred is a plate coated with poly-D-lysine/laminin. The cell population per floating aggregate which adheres to the plate is the one that is the most highly efficient. Preferably, a floating aggregate consists of 200-400 cells.

Examples of the stem cells useful in a preferred embodiment of the present invention include, but are not limited to, bone marrow stem cells (BMSC), cord blood stem cells, amniotic fluid stem cells, fat stem cells, retinal stem cells (RSC), intraretinal Muller glial cells, embryonic stem cells (ESC), induced pluipotent stem cells (iPSC) and somatic cell nuclear transfer cells (SCNTC), with the greatest preference being for human ESC or iPSC. In an embodiment, iPSC as well as human ESC was successfully induced to differentiate into retinal cells including photoreceptor cells by the differentiation method of the present invention.

As used herein, the term “eye field precursor” refers to a cell expressing a marker (eye field transcription factors; Zuber, et al., Development, 2003; 130: 5155-67) found in a progenitor for the eye field of the forebrain neural plate. The eye field precursors are characterized by at least one, two or three markers selected from among Six3, Rax, Pax6, Otx2, Lhx2, and Six6.

As used herein, the term “floating aggregate” refers to a cell mass floating in a medium which is generated when a floc of stem cells is cultured for at least one day in a non-adhesive plate without feeding mouse embryonic fibroblasts and sera. Depending on the composition of the medium supplied, the eye field precursors may express eye field transcription factors.

As used herein, the term “Wnt signaling pathway inhibitor” is intended to refer to a factor which interrupts interaction between the extracellular Wnt protein and the membrane protein Frizzled receptor or LRP or inhibits intracellular Wnt-mediated signal transduction (Kawano & Kypta, J Cell Sci. 2003; 116: 2627-34). So long as it inhibits Wnt-mediated signal transduction, any Wnt signaling pathway inhibitor may be used in the present invention. Examples of the Wnt signaling pathway inhibitors useful in the present invention include the Dkk (Dickkopf) family (Dkk-1, Dkk-2, Dkk-3 and Dkk-4), which are Wnt antagonists capable of interacting with the co-receptor LRP, Wise, the sFRP (secreted Frizzled-related protein) family, which functions as Wnt antagonists binding to Wnt receptors, a Frizzled-CRD domain, WIF-1 (Wnt inhibitory factor-1), IWP-2, IWP-3, IWP-4, cerberus, Wnt antibodies, dominant negative Wnt proteins, overexpression of Axin, overexpression of GSK (glycogen synthase kinase), dominant negative TCF, dominant negative dishevelled and casein kinase inhibitors (CKI-7, D4476 etc.), with preference for Dkk-1.

Wnt signal transduction may be inhibited by suppressing each component involved in the Wnt pathway with for example RNAi, in addition to the Wnt signaling pathway inhibitor.

In a preferred embodiment, IGF-1 or IGF-2 may be used as an IFG1R activator, with priority given to IGF-2. The medium useful for inducing the neural retinal progenitor cells to differentiate into photoreceptor cell precursors contains IGF1R activator in an amount of from 0.01 to 100 ng/ml, preferably in an amount of from 0.1 to 50 ng/ml, more preferably in an amount of from 1 to 20 ng/ml, and most preferably in an amount of 10 ng/ml.

In a preferred embodiment, the BMP signal pathway inhibitor include noggin, chordin, twisted gastrulation (Tsg), cerberus, coco, gremlin, PRDC, DAN, dante, follistatin, USAG-1, dorsomorphin and sclerostin, with preference for noggin. The medium contains the BMP signaling pathway inhibitor in an amount of from 0.01 to 100 ng/ml, preferably in an amount of from 0.1 to 50 ng/ml, more preferably in an amount of from 0.5 to 20 ng/ml, and most preferably in an amount of 10 ng/ml.

As the FGF signaling pathway activator, a factor activating FGRR1c or FGFR3c, FGF1, FGF2, FGF4, FGF8, FGF9, FGF17 or FGF19 may be used, with FGF2 being preferred. The medium useful for inducing the retinal progenitor cells to differentiate into neural retinal progenitor cells contains the FGF signaling pathway activator in an amount of from 0.01 to 100 ng/ml, preferably in an amount of from 0.1 to 50 ng/ml, more preferably in an amount of from 1 to 20 ng/ml, and most preferably in an amount of 5 ng/ml.

In a preferred embodiment, examples of the Wnt signaling pathway activator useful in the present invention include Wnt1, Wnt2, Wnt2b, Wnt3, Wnt3a, Wnt4, Wnt5a, Wnt5b, Wnt6, Wnt7a, Wnt7b, Wnt8a, Wnt8b, Wnt9a, Wnt9b, Wnt10a, Wnt10b, Wnt11, Wnt16b; substances increasing β-catenin levels; GSK3 inhibitors such as lithium, LiCl, bivalent zinc, BIO, SB216763, SB415286, CHIR99021, QS11 hydrate, TWS119, Kenpaullone, alsterpaullone, indirubin-3′-oxime, TDZD-8 and Ro 31-8220 methanesulfonate salt; Axin inhibitors, APC inhibitors, norrin and R-spondin 2, with preference for Wnt3a, Wnt1, Wnt5a, Wnt11, norrin, LiCl, BIO and SB415286. The medium useful for inducing the neural retinal progenitor cells to differentiate into photoreceptor cell precursors contains the Wnt signaling pathway activator except for LiCl, BIO and SB415286 in an amount of from 0.01 to 500 ng/ml, preferably in an amount of from 0.1 to 200 ng/ml, and more preferably in an amount of from 1 to 100 ng/ml. Among the Wnt signaling pathway activators, LiCl is used in the medium in an amount of 0.1 to 50 mM, preferably in an amount of 0.5 to 10 mM, and more preferably in an amount of 1 to 10 mM; BIO is used in an amount of 0.1 to 50 μM, preferably in an amount of 0.1 to 10 μM, and more preferably in an amount of 0.5 to 5 μM; SB415286 is used in an amount of 0.1 to 500 μM, preferably in an amount of 1 to 100 μM, and more preferably in an amount of 5 to 50 μM. In a modification of the embodiment, the medium may contain 50 ng/ml of Wnt3a or Wnt1; 50 or 100 ng/ml of Wnt5a and Wnt11; 50 ng/ml of norrin; 2.5 or 5 mM of LiCl; 2 μM of BIO, or 30 μM of SB415286. In a preferred embodiment, the Shh signaling pathway activator useful in the present invention may be Shh, an inhibitor of Ptc's interaction with Smo, an Smo receptor activator, a substance increasing Ci/Gli family levels, an inhibitor of the intracellular degradation of Ci/Gli factors, or an Shh receptor activator (e.g. Hg—Ag, purmorphamine, etc.). Preferred is Shh or purmorphamine. In a preferred embodiment, the medium contains the Shh signaling pathway activator in an amount of from 0.1 to 5,000 ng/ml, preferably in an amount of from 1 to 2,500 ng/ml, more preferably in an amount of from 10 to 1,000 ng/ml, and most preferably in an amount of 250 ng/ml. In an embodiment of the present invention, the medium contains Shh in an amount of 250 ng/ml or purmorphamine in an amount of 1 μM.

As for the period of time for culturing after differentiation starts, it is preferably given 1-30 days for step (a′), 5-15 days for step (b′), 1-30 days for step (a), 1-30 days for step (b), and 1-60 days for step (c), and most preferably 4 days for step (a′), 9 days for step (b′), 5 days for step (a), 3 days for step (b), and 8-15 days for step (c). In a preferred embodiment, it takes as short as approximately 29 days to complete the differentiation of stem cells into retinal cells, allowing the method to be effectively applied to clinical treatment.

Differentiation into eye field precursors in step (a′) may be accomplished by inducing and promoting the development of the forebrain during embryogenesis with the concomitant suppression of Wnt and BMP signaling pathways (Piccolo, et al., Nature. 1999; 397: 707-10). Therefore, the culture medium contains Dkk-1 as a Wnt signaling pathway inhibitor, noggin as a BMP inhibitor, and IGF-1 as an IGF1R activator functioning to promote the formation of the eye field in the forebrain. Kinds and medium levels of the Wnt signaling pathway inhibitor, the BMP inhibitor and IGF1R activator are as defined above.

The medium useful in step (a′) contains IGF-1 at a concentration of 0.01-100 ng/ml, Dkk-1 at a concentration of 0.01-10,000 ng/ml and noggin at a concentration of 0.01-100 ng/ml, and most preferably IGF-1 at a concentration of 5 ng/ml, Dkk-1 at a concentration of 1 ng/ml, and noggin at a concentration of 1 ng/ml.

Any conventional medium for culturing stem cells may be used for inducing differentiation into eye field precursors in step (a′). Preferred is DMEM/F12 containing 10% knockout serum replacement, 1 mM L-glutamine, 0.1 mM non-essential amino acids, 0.1 mM mercaptoethanol, and 1% B27 supplement.

Differentiation into retinal progenitor cells in step (b′) may be conducted in a medium containing an FGF signaling pathway activator, preferably FGF2, in combination with the factors of step (a′), that is, an IGF1R activator, a Wnt signaling pathway inhibitor and a BMP inhibitor.

The medium useful in step (b′) contains IGF-1 at a concentration of 0.01-100 ng/ml, Dkk-1 at a concentration of 0.01-10,000 ng/ml, noggin at a concentration of 0.01-100 ng/ml and FGF2 at a concentration of 0.01-100 ng/ml and most preferably IGF-1 at a concentration of 10 ng/ml, Dkk-1 at a concentration of 10 ng/ml, noggin at a concentration of 10 ng/ml and FGF2 at a concentration of 5 ng/ml.

The differentiation of the retinal progenitor cells of step (b′) into neural retinal progenitor cells in step (a), must be conducted in the absence of Dkk-1, a Wnt signaling pathway inhibitor, for there to be a high level of Pax6 expressed (Pax6 is essential for the generation of neural retinal progenitor cells), and in a medium containing Wnt3a for promoting the Wnt pathway, noggin for converting ventral retinal pigmented epithelium into the neural retina in the development stage of optic vesicles and optic cups during embryogenesis, FGF2 for suppressing the expression of genes necessary for retinal pigmented epithelium and promoting the generation of neural retinal progenitor cells, and IGF-1 responsible for the antiapoptosis of the photoreceptor cells.

The medium useful in step (a) contains IGF-1 in a concentration of 0.01-100 ng/ml, noggin in a concentration of 0.01-100 ng/ml, FGF2 in a concentration of 0.01-100 ng/ml, and Wnt3a in a concentration of 0.01-500 ng/ml and most preferably IGF-1 in a concentration of 10 ng/ml, noggin in a concentration of 10 ng/ml, FGF2 at a concentration of 5 ng/ml and Wnt3a at a concentration of 50 ng/ml.

Differentiation into photoreceptor cell precursors in step (b) is conducted in a medium which is free of both noggin and FGF2 that respectively inhibit Shh signaling pathway and Shh-induced rhodopsin expression, and which contains IGF-1 for proliferating rod photoreceptor cell precursors, Wnt3a for promoting the Wnt pathway, and Shh.

The medium useful in step (b) contains IGF-1 in a concentration of 0.01-100 ng/ml, Wnt3a at a concentration of 0.01-500 ng/ml and Shh at a concentration of 0.1-5,000 ng/ml and most preferably IGF-1 at a concentration of 10 ng/ml, Wnt3a at a concentration of 50 ng/ml and Shh at a concentration of 250 ng/ml.

Differentiation into photoreceptor cells in step (c) is conducted in a medium which contains RA (retinoic acid) for further promoting the differentiation in combination with IGF-1, Wnt3a and Shh.

The medium useful in step (c) contains IGF-1 in a concentration of 0.01-100 ng/ml, Wnt3a at a concentration of 0.01-500 ng/ml, Shh at a concentration of 0.01-5,000 ng/ml and RA at a concentration of 0.5-10,000 nM and most preferably IGF-1 at a concentration of 10 ng/ml, Wnt3a at a concentration of 50 ng/ml, Shh at a concentration of 250 ng/ml and RA at a concentration of 500 nM.

In steps (a) to (c), Wnt1, Wnt5a, Wnt11, norrin, LiCl, BIO or SB415286 may be used instead of Wnt3a while purmorphamine may substitute for Shh.

Any conventional medium for culturing embryonic stem cells may be used as a fundamental medium in steps (b′), (a), (b) and (c). Preferred is DMEM/F12 containing 1 mM L-glutamine, 0.1 mM non-essential amino acids, 0.1 mM mercaptoethanol, 1% B27 supplement and 1% N2 supplement.

In an embodiment of the present invention, a Wnt signaling pathway activator was observed to play a critical role in differentiating retinal progenitor cells into photoreceptor cells. In this regard, differentiation was conducted in the presence of various concentrations of the Wnt signaling pathway inhibitor Dkk-1 (10 ng/ml, 100 ng/ml and 1 μg/ml). In detail, hESC-derived retinal progenitor cells were induced to differentiated into photoreceptor cells in a medium containing 50 ng/ml Wnt3a, but neither Shh nor retinoic acid (group I), containing neither Wnt3a nor Dkk-1 (group II), containing 10 ng/ml Dkk-1 (group III), containing 100 ng/ml Dkk-1 (group IV) and containing 1 μg/ml Dkk-1 (group V).

As a result, the photoreceptor cell markers Crx (p=0.0247), recoverin (p=0.0113), rhodopsin (p=0.0166) and peripherin2 (p=0.0166) remarkably decreased in expression level with increasing of Dkk-1 concentration. Statistical significances were found between all groups except for group I vs. group II and group IV vs. group V in Crx and group I vs. group II in recoverin (significance between groups: *: p<0.0001; **: p=0.0381) (Table 10 and FIG. 24). These data showed that the differentiation into photoreceptor cells is induced by a Wnt signaling pathway activator and that the inhibition of the Wnt signaling pathway leads to almost no generation of photoreceptor cells, giving evidence of the Wnt signaling pathway inhibitor playing a critical role in differentiation into photoreceptor cells.

On the other hand, respective substitutes may be used instead of Wnt3a and Shh. In this context, the culturing processes are carried out for the same time period under the same conditions with the exception that 50 ng/ml of Wnt1; 50 and 100 ng/ml of Wnt5a and Wnt11; 50 ng/ml of norrin; 2.5 or 5 mM of LiCl; 2 μM of BIO; or 30 μM of SB415286 is used as a Wnt signaling pathway activator in substitution for Wnt3a and purmorphamine is used as a substitute for Shh. These substitutes, as a result, give rise to the differentiation and maturation of photoreceptor cells in a similar pattern to that obtained with Wnt3a and Shh (Example 12).

MODE FOR INVENTION

A better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed as limiting the present invention.

Example 1 Culture of Stem Cells <1-1> Culture of Human Embryonic Stem Cells

The human embryonic stem cell (hESC) lines H9 (WA09, normal karyotype XX) and H7 (WA07, normal karyotype, XX) were purchased from the WiCell Research Institute (Madison, Wis., USA).

The hESCs were allowed to proliferate undifferentiated (H9 cells: passages 25^(˜)33; H7 cells: passages 23^(˜)28) by culturing over feeder cells, such as radiated mouse embryonic fibroblasts (MEF, ATCC, Manassas, Va., USA) or mitomycin-treated mouse embryonic fibroblasts (EmbryoMax Primary Mouse Embryo Fibroblasts, Millipore, Billerica, Mass., USA) in the following medium: DMEM/F12 (Invitrogen, Grand Island, N.Y., USA), 20% (v/v) KnockOut serum replacement, Invitrogen, Carlsbad, Calif., USA), 1 mM L-glutamine (Invitrogen), 0.1 mM nonessential amino acids (Invitrogen), 0.1 mM mercaptoethanol (Sigma-Aldrich, St. Louis, Mo., USA), and 4 ng/ml human recombinant basic fibroblast growth factor (FGF2, Invitrogen).

While the medium was replaced with a fresh one every day, the undifferentiated stem cells were passaged at a ratio of 1:9-1:15 every six or seven days manually or with collagenase IV (Invitrogen), and then transferred onto fresh MEF feeder cells. During the passage of the hESCs, immunochemical staining with OCT-4 and SSEA-4 (Chemicon, Temecula, Calif., USA), antigens characteristic of undifferentiated hESCs, was conducted at regular intervals of time to monitor the degree of differentiation. Cells that were found to have undergo differentiation were removed.

The presence of mycoplasma contamination in the hESC culture, which could have an undesirable effect on the differentiation of hESCs, was regularly monitored with a kit (MycoAlert mycoplasma detection kit, Lonza, Rockland, Me., USA).

<1-2> Culture of Induced Pluipotent Stem Cells (iPSCs)

The human iPSC line iPS (Foreskin)-1 (Clone 1) (normal karyotype XY) was purchased from the WiCell Research Institute (Madison, Wis., USA).

The human iPSCs were allowed to proliferate undifferentiated (passages 37-47) by culturing them over feeder cells, such as radiated mouse embryonic fibroblasts (MEF, ATCC, Manassas, Va., USA) or mitomycin-treated mouse embryonic fibroblasts (EmbryoMax Primary Mouse Embryo Fibroblasts, Millipore, Billerica, Mass., USA) in the following medium: DMEM/F12 (Invitrogen, Grand Island, N.Y., USA), 20% (v/v) KnockOut serum replacement (Invitrogen, Carlsbad, Calif., USA), 1 mM L-glutamine (Invitrogen), 0.1 mM nonessential amino acids (Invitrogen), 0.1 mM mercaptoethanol (Sigma-Aldrich, St. Louis, Mo., USA), and 10 ng/ml human recombinant FGF2 (Invitrogen).

While the medium was replaced with a fresh one every day, the undifferentiated stem cells were passaged at a ratio of 1:4-1:6 every six or seven days manually or with collagenase IV (Invitrogen), and then transferred onto fresh MEF feeder cells. During the passage of the hESCs, immunochemical staining with SSEA-4 (Chemicon, Temecula, Calif., USA) and Nanog (abcam, Cambridge, Mass., USA), which are antigens characteristic of undifferentiated human iPSCs, was conducted at regular intervals of time to monitor the degree of differentiation. Cells that were identified to have undergone differentiation were removed.

The presence of mycoplasma contamination in the hESC culture, which could have an undesirable effect on the differentiation of hESCs, was regularly monitored with a kit (MycoAlert mycoplasma detection kit, Lonza, Rockland, Me., USA).

Example 2 Differentiation from hESCs or Human iPSCs into Eye Field Precursors

The hESCs or human iPSCs cultured in Example 1 were separated from the MEF cells (FIGS. 1 and 14) and seeded into 6-well ultra-low attachment plates (Corning Incorporated, Corning, N.Y., USA).

To the hESCs or human iPSCs in the 6-well ultra-low attachment plates was added a medium for inducing differentiation into eye field precursors [DMEM/F12, 10% KnockOut serum replacement, 1 mM L-glutamine, 0.1 mM nonessential amino acids, 0.1 mM mercaptoethanol, 1% B27 supplement (Invitrogen), 1 ng/ml recombinant noggin (R&D Systems), 1 ng/ml recombinant Dkk-1 (Dickkopf-1, R&D Systems), and 5 ng/ml recombinant IGF-1 (insulin-like growth factor-1, R&D Systems)]. The cells were cultured for 4-5 days to generate eye field precursors in the form of floating aggregates with the replacement of the medium with a fresh one every third day (FIG. 1).

Example 3 Differentiation from Eye Field Precursors into Retinal Progenitor Cells

The eye field precursors (floating aggregates) generated in Example 2 were seeded at a density of 53±8 cells per well (292±53 cells/floating aggregate) into 6-well poly-D-lysine/laminin-coated plates (BD Biosciences) and at a density of 12±4 cells per well on 8-well poly-D-lysine/laminin-coated plates and then cultured for 9 days to generate retinal progenitor cells, with the supply of a medium for inducing differentiation into retinal progenitor cells [DMEM/F12 (Invitrogen), 1 mM L-glutamine (Invitrogen), 0.1 mM nonessential amino acids (Invitrogen), 0.1 mM mercaptoethanol (Sigma-Aldrich), 1% B27 supplement, 1% N2 supplement (Invitrogen), 10 ng/ml Dkk-1, 10 ng/ml noggin, 10 ng/ml IGF-1, and 5 ng/ml FGF2].

Example 4 Differentiation from Retinal Progenitor Cells into Neural Retinal Progenitor Cells

To differentiate the retinal progenitor cells generated in Example 3 into neural retinal progenitor cells, Dkk-1 supply was quit on day 3 after induction and a culture medium containing a combination of noggin, IGF-1, FGF2 and Wnt3a was supplied for 5 days (FIG. 1). The culture medium was composed as follows: [DMEM/F12 (Invitrogen), 1 mM L-glutamine (Invitrogen), 0.1 mM nonessential amino acids (Invitrogen), 0.1 mM mercaptoethanol (Sigma-Aldrich), 1% B27 supplement (Invitrogen), 1% N2 supplement (Invitrogen), 10 ng/ml noggin, 10 ng/ml IGF-1, 5 ng/ml FGF2, and 50 ng/ml recombinant Wnt3a (R&D Systems)].

Example 5 Differentiation from Neural Retinal Progenitor Cells into Photoreceptor Cell Precursors

The neural retinal progenitor cells generated in Example 4 were induced to differentiate into photoreceptor cell precursors by supplying a medium [DMEM/F12 (Invitrogen), 1 mM L-glutamine (Invitrogen), 0.1 mM nonessential amino acids (Invitrogen), 0.1 mM mercaptoethanol (Sigma-Aldrich), 1% B27 supplement (Invitrogen), 1% N2 supplement (Invitrogen), 10 ng/ml IGF-1, 50 ng/ml Wnt3a, and 250 ng/ml recombinant Shh (Sonic Hedgehog amino terminal peptide, Shh, R&D Systems)] for 3 days. Noggin and FGF2, both used in the previous step, were excluded from the medium (FIG. 1).

Example 6 Differentiation from Photo Receptor Cell Precursors into Photo Receptor Cells

A specialized medium [DMEM/F12 (Invitrogen), 1 mM L-glutamine (Invitrogen), 0.1 mM nonessential amino acids (Invitrogen), 0.1 mM mercaptoethanol (Sigma-Aldrich), 1% B27 supplement (Invitrogen), 1% N2 supplement (Invitrogen), 10 ng/ml IGF-1, 50 ng/ml Wnt3a, 250 ng/ml Shh, 500 nM (all-trans retinoic acid (RA, Sigma-Aldrich)] were supplied for 8 days or longer to induce the photoreceptor cell precursors to differentiate into photoreceptor cells.

All the media were replaced with fresh ones every two or three days in Examples 2 to 6, and the cells were cultured at 37° C. in a 5% CO₂ atmosphere. All induction and differentiation experiments were repeated at least three times and the same results were obtained therefrom.

Example 7 Assay for Cellular Differentiation-Related Markers <7-1> Immuno Chemical Staining and Identification of Cellular Differentiation-Related Marker Expression

The differentiation of the cells obtained in Examples 3 to 6 was examined using an immunochemical staining method as follows.

The eye field precursors (floating aggregates) were cultured in 8-well poly-D-lysine/laminin-coated slides (BD Biosciences, Bedford, Mass.) under the same conditions that were used for differentiation into the retinal progenitor cells, the neural retinal progenitor cells, the photoreceptor cell precursors and the photoreceptor cells. The cells completely cultured in each step were fixed with 4% paraformaldehyde (Sigma-Aldrich), after which non-specific reactions were blocked with PBS containing 3% BSA (Jackson Immunoresearch Laboratory, Bar Harbor, Me., USA) and 0.25% Triton X-100 (Sigma-Aldrich).

After being blocked for 90 min, the slides in each differentiation step were incubated overnight at 4° C. with the following antibodies specific for cells of each differentiation step: rabbit-Blue-opsin (1:500, Chemicon), sheep-Chx10 (1:100, Chemicon), rabbit-Crx (1:200, Santa Cruz Biotechnology, Santa Cruz, Calif., USA), rabbit-GFAP (1:200, Invitrogen), mouse-human specific mitochondria (1:50, Chemicon), rabbit-human specific mitochondria (1:200, Chemicon), mouse-Islet1 (1:10, Developmental Studies Hydroma Bank, DSHB; Iowa City, Iowa, USA), mouse-Ki67 (1:100, Vector Laboratories, Peterborough, England), mouse-Mitf (1:1,000, abcam, Cambridge, Mass., USA), mouse-nestin (1:250, BD Sciences), rabbit-neurofilament-200 (1:1,000, Sigma-Aldrich), rabbit-Otx2 (1:100, abcam), rabbit-Pax2 (1:250, abcam), mouse-Pax6 (1:2, DSHB), mouse-peripherin2 (1:500, GenScript, Piscataway, N.J., USA), rabbit-Rax (1:250, abcam), rabbit-recoverin (1:1,000, Chemicon), retinal pigment epithelium 65 (RPE65, 1:100, Chemicon), rabbit-rhodopsin (1:500, Sigma-Aldrich), mouse-rhodopsin (1:500, Ret-P1, Lab Vision, Fremont, Calif., USA), mouse-rhodopsin (1:2,000, Ret-P1, Sigma-Aldrich), mouse-rom1 (1:50, ABR-Affinity Bioreagents, Golden, Colo., USA), rabbit-PDE6 beta (1:100, abcam), rabbit-phosducin (1:500, Santa Cruz Biotechnology), mouse-PKC-alpha (1:500, Sigma-Aldrich), mouse-Prox1 (1:2,000, Chemicon), mouse-Sox2 (1:250, R&D Systems), rabbit-synaptophysin (1:2,000, Santa Cruz Biotechnology), and rabbit-ZO-1 (1:100, Zymed-Invitrogen).

Before use, these antibodies were diluted in a PBS solution containing 1% BSA and 0.25% Triton X-100. The cells cultured on the slides in each step were washed three times for 5 min with PBS and incubated at room temperature for 2 hrs with species-specific secondary antibody conjugated with Cy3 (1:800, Jackson Immunoresearch Laboratory) or Alexa488 (1:500, Invitrogen). A standard material suitable for the primary and the secondary antibody was used to examine non-specific staining or interaction between the antibodies. Afterwards, the cells were washed three times for 5 min with PBS, counterstained with DAPI (4,6-diamidino-2 phenylindole) and mounted in Vectashield (Vector Laboratories), followed by visualization under an epifluorescence microscope (Nikon Eclipse, E800, Tokyo, Japan) and a confocal microscope (Leica, Leica Microsystems Inc, Bannockburn, Ill., USA or Zeiss LSM510, Carl Zeiss, Inc, Thornwood, N.Y., USA).

500 cells were counted from 20 microscopic fields randomly selected at 200 magnification and evaluated for positive responses to each antibody. Positive responses to antibodies were determined after at least three evaluations. Statistical analysis of the data was done using the Kruskal-Wallis test and the Bland-Altman plot (Bland and Altman, 1986) of Med Calc version 8.1.1.0 as well as the GEE (Generalized Estimating Equations) model of SAS version 9.1. All data were represented as mean±standard error of the mean (S.E.M) with a statistical significance of p<0.05.

With regard to the markers characteristic of both forebrain eye field precursors and retinal progenitor cells, Rax was found to be expressed on the retinal progenitor cells generated in Example 3 at a positive rate of 86.6±3.0%, Pax6 at a positive rate of 63.9±0.9%, Otx2 at a positive rate of 76.4±2.0%, Sox2 at a positive rate of 83.0±1.9%, and Chx10 at a positive rate of 46.3±1.0%. The cells were also found to express the marker Mitf characteristic of retinal pigmented epithelial progenitor cells at a positive rate of 17.2±0.4%, and the marker nestin characteristic of neural progenitor cells at a positive rate of 65.7±2.7% (Tables 1 and 2).

TABLE 1 Change in Marker Level with Culture Time Period Mean ± S.E.M. (%, Sample size = 3) Marker 13 Days 18 Days 21 Days 30 Days Rax 86.6(±3.0) 98.2(±0.9) 97.6(±0.6) 97.8(±0.7) Pax6 63.9(±0.9) 89.1(±2.5) 61.6(±0.5) 89.7(±1.9) Otx2 76.4(±2.0) 61.5(±0.6) 63.3(±2.9) 57.2(±2.4) Sox2 83.0(±1.9) 68.1(±1.1) 49.1(±1.6) 69.1(±4.0) Chx10 46.3(±1.0) 64.5(±1.6) 48.5(±3.9) 41.6(±2.0) Nestin 65.7(±2.7) 18.0(±1.4) 14.3(±2.3)  8.5(±0.9) Mitf 17.2(±0.4)  2.7(±1.0) 18.6(±1.3) 24.7(±0.3)

On the neural retinal progenitor cells generated in Example 4, the positive rates were increased simultaneously from 63.9% to 89.1% for Pax 6, from 86.6% to 98.2% for Rax, and from 46.3% to 64.5% for Chx10 (p<0.0001), indicating that the increase in the positive rate of Pax6 resulted from the proliferation of neural retinal progenitor cells (both Rax+ and Pax6+ positive) (Tables 1 and 2).

TABLE 2 Statistical Significance of Retinal Cell Markers between Culture Periods of Time Statistical Significance (p values) 13 Days vs. 18 Days vs. 21 Days vs. Marker 18 Days 21 Days 30 Days Rax <0.0001 0.5308 0.7897 Pax6 <0.0001 <0.0001 <0.0001 Otx2 <0.0001 0.4596 0.0505 Sox2 <0.0001 <0.0001 <0.0001 Chx10 <0.0001 <0.0001 0.0531 Nestin <0.0001 0.1077 0.0010 Mitf <0.0001 <0.0001 0.0134 *Statistical significance: p < 0.05

As shown in Tables 1 and 2, the positive rate of the neural progenitor cell marker nestin greatly decreased from 65.7% to 18.0% (p<0.0001), from which it is understood that on differentiation day 18, the proliferation of neural progenitor cells was restrained while the proliferation and differentiation of neural retinal cells were promoted.

On the other hand, the proliferative cell marker Ki67 rapidly decreased from 87.5% to 31.5% (p<0.0001), indicating that differentiation was more active than proliferation (FIG. 2 and Tables 3 and 4).

TABLE 3 Change in Expression Level of Retinal Cell Markers with Culture Period of Time Mean ± S.E.M. (%, sample size = 3) Marker 13 Days 18 Days 21 Days 30 Days Crx 12.8(±1.6) 80.1(±0.2) 54.8(±4.2) 39.5(±7.4) Recoverin 35.8(±0.4) 68.5(±2.6) 61.3(±1.8) 82.4(±4.6) Rhodopsin 35.3(±1.7) 52.9(±3.4) 60.4(±3.2) 81.2(±2.5) Peripherin2  5.6(±0.3) 13.9(±2.4) 26.0(±0.4) 41.2(±2.0) Ki67 87.5(±1.3) 31.5(±0.5) 58.4(±4.1) 30.2(±6.1)

TABLE 4 Statistical Significance of Retinal Cell Markers between Culture Periods of Time Statistical Significance (p values) 13 Days vs. 18 Days vs. 21 Days vs. Marker 18 Days 21 Days 30 Days Crx <0.0001 <0.0001 0.0326 Recoverin <0.0001 0.0079 <0.0001 Rhodopsin <0.0001 0.0489 <0.0001 Peripherin2 <0.0001 <0.0001 <0.0001 Ki67 <0.0001 <0.0001 <0.0001 *Statistical significance: p < 0.05

The positive rate of the photoreceptor cell precursor marker Crx showed a drastic increase from 12.8% before Wnt3a addition to 80.1% after Wnt3a addition (p<0.0001) (FIG. 2 and Tables 3 and 4). An increase in the expression level from 35.8% to 68.5% (p<0.0001) was also found in the universal photoreceptor cell marker recoverin, in the rod photoreceptor cell marker rhodopsin from 35.3% to 52.9% (p<0.0001), and in the photoreceptor cell's outer segment marker peripherin2 from 5.6% to 13.9% (p<0.0001) (FIG. 2, Tables 3 and 4). Mitf, an antigen marker of early pigmented epithelial progenitor cells (Baumer, et al., Development. 2003; 130: 2903-15), showed a decrease in positive rate from 17.2% to 2.7% (p<0.0001) (Tables 1 and 2). Also, a decrease in positive rate was found in the retinal progenitor cell markers Otx2 (p<0.0001) and Sox2 (p<0.0001), implying differentiation into neural retinal progenitor cells.

In the photoreceptor cell precursors generated in Example 5, decreased expression levels were detected for the markers of both retinal progenitor cells and neural retinal progenitor cells, including Pax6 (from 89.1% to 61.6%, p<0.0001), Chx10 (from 64.5% to 48.5%, p<0.0001) and Sox2 (from 68.1% to 49.1%, p<0.0001) (Tables 1 and 2). The photoreceptor cell precursor marker Crx also decreased from 80.1% to 54.8% (p<0.0001) (FIG. 2, Tables 3 and 4). On the other hand, an increase in expression level was detected in the photoreceptor cell marker rhodopsin (from 52.9% to 60.5%, p=0.0489) and in the photoreceptor cell's outer segment marker peripherin2 (from 13.9% to 26.0%, p<0.0001), indicating the differentiation and maturation of photoreceptor cells (FIG. 2, Tables 3 and 4). Further, the decreasing positive rate of Ki67 was reversed into increasing from 31.5% to 58.4% (p<0.0001) (FIG. 2, Tables 3 and 4), from which it is understood that Shh started to promote cell proliferation.

As high as 82.4±4.6% of the population of cells generated by the method of Example 6 for differentiation into photoreceptor cells were found to react positive to recoverin, a universal marker of photoreceptor cells (rod photoreceptor cells and cone photoreceptor cells), as measured by a quantitative antigen assay (FIG. 3). Also, the quantitative antigen assay showed that 81.2±2.5% of the cell population was positive to rhodopsin, an antigen characteristic of rod photoreceptor cells (FIG. 3). The fact that almost all of the rhodopsin-positive cells show a positive reaction to recoverin ensures the reliability of the positive responses.

More precise determination of the existence and integrity of the rhodopsin molecules formed by the method of the present invention was carried out using human-specific recombinant rhodopsin (consisting of the amino acids of the second extracellular loop in human rhodopsin) and Ret-P1 (consisting of N-terminal amino acids at positions 4 to 10 in rhodopsin), both of which are rhodopsin antibodies which recognize different epitopes.

The human-specific recombinant rhodopsin and the Ret-P1 were found to have identical positive rates, with a statistically significant difference therebetween, as measured by the Bland-Altman plot (average of difference in positive rate between the two antibodies: −1.00, 95% confidence interval: −13.6−11.6). The cells reacted with almost no difference in the positive rate with both human-specific recombinant rhodopsin and Ret-P1 antibodies. This implies that the rhodopsin molecule formed according to the method of the present invention retains two different epitopes therein.

The rod photoreceptor cell's outer segment markers peripherin2 (Prph2) and rom1 (retinal outer segment membrane protein 1) were positively detected in 41.2±2.0% and 76.0±4.6% of the population of the cells cultured, respectively (FIG. 4). Both of these two markers were observed only in rhodopsin-positive cells (FIG. 4). Positive rates in the photoreceptor cells were measured to be 49.8±2.2% for phosducin, which responds to light, that is, participates in the regulation of visual phototransduction (FIG. 5) and 43.0±2.0% for synaptophysin which is responsible for synaptic interactions with other intraretinal neurons (FIG. 6). These facts prove that these cells participate in the neural circuits of the retina and perform the function of transducing light stimuli into neural electric stimuli. Taken together, the data obtained above demonstrate that the method of the present invention can differentiate with high efficiency human embryonic stem cells into photoreceptor cells.

Blue opsin-cone photoreceptor cells were observed in 80.2±0.6% of all the cells (FIG. 7). These cells were found from rhodopsin-positive cell flocs or in the vicinity thereof, some of which were positive to both rhodopsin and blue opsin. Accordingly, the method of the present invention was proven capable of inducing human embryonic stem cells to differentiate into both rod and cone photoreceptor cells.

Of the total population of the cells, 39.5±7.4% were positive to Crx, a marker characteristic of photoreceptor cell precursors, and 30.2±6.1% were positive to Ki67, a nuclear marker of proliferating cells (late G1 phase to M phase in the cell cycle). Photoreceptor cell precursors are reported to express Crx immediately after leaving the cell proliferation cycle. Also in the present invention, most of the human ESC-derived, Crx-positive cells were observed to be negative to K167, a marker associated with cell proliferation. However, the Crx-positive cells were found to express Ki67 at a positive rate of 5.5±1.7% (FIG. 9).

From the data obtained above, it is apparent that the method of the present invention can effectively differentiate photoreceptor cells from human ESC-derived retinal progenitor cells in the same multi-step induction pattern as they do from embryonic retinal progenitor cells, where differentiation progresses in steps along the early and late embryonic stage. The retinal progenitor cell marker Pax6 had a positive rate of 89.7±1.9% and was detected in amacrine cells, which are a type of differentiated retinal neurons. Meanwhile, the retinal progenitor cells expressed Mitf (a marker of retinal pigmented epithelium (RPE) progenitor cells) at a positive rate of 24.7±0.3%, and ZO-1 (a marker of differentiated RPE) at a positive rate of 12.5±1.4%, and were also positive to RPE65, a marker of differentiated RPE (FIG. 10).

<7-2> RT-PCR and RNA mRNA Expression of Cellular Differentiation-Associated Markers

To examine the differentiation of the photoreceptor cells generated in Example 6, a reverse transcriptase-polymerase chain reaction (RT-PCR) was performed on Day 29 of differentiation induction.

Total RNA was isolated with an RNeasy minikit (Qiagen, Valencia, Calif., USA) and DNA was removed from the RNA isolate using DNaseI (Applied Biosystems/Ambion, Austin, Tex., USA). 500 g of the RNA was reverse transcribed into cDNA in the presence of reverse transcriptase (Omniscript reverse transcriptase, Qiagen), with random hexamers (Invitrogen) serving as primers. For the PCR, 1×PCR buffer, 1.5 mM MgCl₂, 0.2 mM dNTPs, 50 ng DNA, 0.5 U AmpliTaq Gold (Applied Biosystems, Foster City, Calif., USA) and 10 pmoles of each of the primers given in Table 5, below, were used.

TABLE 5 Forward and Reverse Primer Sequences for RT-PCR SEQ PCR ID Primer Sequence Product Gene NO. (5′ to 3′) Size (bp) PAX6 1 Foward: 275 aacagacacagccctcacaaaca PAX6 2 Reverse: cgggaacttgaactggaactgac SIX3 3 Foward: 307 cgggagtggtacctacagga SIX3 4 Reverse: ttaccgagaggatggaggtg SIX6 5 Foward: 272 cctgcaggatccatacccta SIX6 6 Reverse: tgatggagatggctgaagtg LHX2 7 Foward: 285 ccaaggacttgaagcagctc LHX2 8 Reverse: tgccaggcacagaagttaag RAX 9 Foward: 495 ggcaaggtcaacctaccaga RAX 10 Reverse: gtgctccttggctttcagac CRX 11 Foward: 353 gtgtggatctgatgcaccag CRX 12 Reverse: tgagatgcccagagggtct Chx10 13 Foward: 281 ggcgacacaggacaatcttt Chx10 14 Reverse: atccttggctgacttgagga NRL 15 Foward: 206 ggcactgaccacatcctctc NRL 16 Reverse: ggaggcactgagctgtaagg RCVRN 17 Foward: 150 agctccttccagacgatgaa RCVRN 18 Reverse: caaactggatcagtcgcaga RHO 19 Foward: 258 taagcccatgagcaacttcc RHO 20 Reverse: agctgcccatagcagaaaaa RBP3 21 Foward: 291 cagcccatatccctgagaat RBP3 22 Reverse: agcacaagatgggaatggag PDE6B 23 Foward: 409 aggagaccctgaacatctacc PDE6B 24 Reverse: atgaagcccacttgcagc OPN1SW 25 Foward: 206 ctgggcactgtagcaggtct OPN1SW 26 Reverse: tgcaggccctcagggatg ASCL1 27 Foward: 467 catctcccccaactactcca ASCL1 28 Reverse: cttttgcacacaagctgcat NEUROD1 29 Foward: 523 gccccagggttatgagactatcact  NEUROD1 30 Reverse: ccgacagagcccagatgtagttctt ATHO7 31 Foward: 246 tcgcatcatcagacctatgg ATHO7 32 Reverse: ccgaacaggacaaactcaca POU4F2 33 Foward: 175 caaccccaccgagcaata POU4F2 34 Reverse: gtgcacgggatggtattcat ARX 35 Foward: 462 tgaaacgcaaacagaggcgcta ARX 36 Reverse: tgatgaaagctgggtgtcggaaca AFP 37 Foward: 318 tttagctgacctggctaccat AFP 38 Reverse: cagcttgtgacaggttctgg T 39 Foward: 541 ccgtctccttcagcaaagtc T 40 Reverse: caattgtcatgggattgcag GAPDH 41 Foward: 302 agccacatcgctcagacacc GAPDH 42 Reverse: gtactcagcgccagcatcg SAG 43 Foward: 400 aaaaagtgccaccaaacagc SAG 44 Reverse: acgtcattcttgtctctcttcc 44 Reverse: acgtcattcttgtctctcttcc

After initial DNA denaturation at 94° C. for 10 min, all PCR was performed over 35 cycles starting with initial DNA denaturation (94° C. for 30 sec, 60° C. for 30 sec, 72° C. for 1 min) and terminating with extension at 72° C. for 10 min. The PCR products thus obtained were isolated by electrophoresis on 2% agarose gel and analyzed.

All experiments were conducted in triplicate and human GAPDH was used as a reference molecule for standard mRNA calculation.

The retinal progenitor cell-related genes RAX, PAX6, SIX3, SIX6, LHX2 and Chx10 were detected in the RT-PCR products. Inter alia, RAX and PAX6 were expressed to as the same high extent as was the quantitative control gene GAPDH. Genes relevant to photoreceptor cells and other retinal cells were also examined for mRNA expression. The PCR products were observed to include the photoreceptor cell-associated genes CRX, NRL, RCVRN, RHO, PDE6B, SAG and OPN1SW, the retinal ganglion cell-related genes ATHO7 and POU4F2, and the amacrine cell-related gene NEUROD1, and the bipolar cell-related gene ASCL1 (FIG. 16).

<7-3> Base Sequencing for Identifying mRNA Expression of Recoverin and Rhodopsin, Both Specific for Differentiated Photoreceptor Cells

In order to investigate the differentiation of the photoreceptor cells generated in Example 6, base sequencing was conducted on the photoreceptor cell-specific genes recoverin (RCVRN: NM_(—)002903.2) and rhodopsin (RHO: NM_(—)000539.3) on Day 29 of differentiation.

The PCR products were purified with a QIAquick 96-well PCR purification kit (Qiagen, Valencia, Calif.) and sequenced using the forward and reverse primers listed in Table 5, with the aid of a base sequencing kit (BigDye terminator cycle sequencing ready reaction kit, Applied Biosystems, Foster City, Calif., USA). The fluorescent labeled fragments thus produced were purified by ethanol precipitation, resuspended in distilled water, and electrophoresed in an ABI PRISM 3700 DNA analyzer (Applied Biosystems, Foster City, Calif., USA), followed by analysis of the base sequences with a sequencer (Gene Codes Corporation, Ann Arbor, Mich., USA).

The RCVRN and RHO genes expressed in the photoreceptor cells differentiated according to the present invention were found to perfectly coincide with human standard sequences (http://www.ncbi.nlm.nih.gov), indicating that the photoreceptor cells express human RCVRN and RHO genes (FIG. 17).

Example 8 Transplantation of Photoreceptor Cells Differentiated from hESCs and Evaluation of Transplanted Cells <8-1> Transplantation of Differentiated Photoreceptor Cells

The transplantability and clinical applicability of the photoreceptor cells generated in Example 6 to blind persons were examined on the immunosuppressant retinal-degeneration mouse model rd/SCID.

After getting permission from the institutional Review Board of the Seoul National University College of Medicine/the Seoul National University Hospital and the Institutional Animal Care and Use Committee (IACUC) of Seoul National University/the Seoul National University Hospital, all animal experiments were conducted according to the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. For the transplantation, four-week-old C3H/Prkdc mice (rd/SCID, Jackson Laboratory, BarHarbor, Me., USA) were employed. C3H/Prkdc mice were also used as a negative control.

The retinal cells comprising photoreceptor cells differentiated from hESCs according to the present invention were detached with accutase and suspended in Dulbecco's PBS (D-PBS, Invitrogen) to form a single-cell suspension with a density of 6-10×10⁴ cells/μL. A surgical procedure was carried out under a dissecting microscope (SZ51, Olympus, Tokyo, Japan). After the mice were anesthetized by intrapeneal injection of a mixture of zoletil (7.5 mg/kg, Virbac Laboratoires, Carros, France) and xylazine (10 mg/kg, BayerKorea, Korea), drops of a cycloplegic-mydriatic agent (Mydrin-P, Santen Pharmaceutical Co., Osaka, Japan) and 0.5% proparacaine (Alcaine 0.5%, Alcon, Inc., Puurs, Belgium) were put into the eyes. 1 μL of the cell suspension was transplanted into each mouse by subretinal injection using a 10 μL injector (NanoFil microsyringe, World Precision Instruments, WPI, Sarasota, Fla., USA) equipped with a 35-gauge needle (WPI).

<8-2> Electroretinography Retinal Degeneration after Transplantation into Mice with Retinal Degeneration

The retinal function of the mice transplanted with the photoreceptor cells differentiated from hESCs according to the present invention was evaluated by electroretinography (ERG, Roland Consult, Wiesbaden, Germany) 3-5 weeks after transplantation. For this, the mice were acclimated to darkness for one day before the test. Prior to the electroretinography, the mice were anesthetized by intraperitoneal injection of a mixture of zoletil (7.5 mg/kg, Virbac Laboratoires, Carros, France) and xylazine (10 mg/kg, BayerKorea, Korea) and drops of Mydrin-P (Santen Pharmaceutical Co., Osaka, Japan) and 0.5% proparacaine (Alcaine 0.5%, Alcon, Inc., Puurs, Belgium) were applied to the eyes. Simultaneously, vidisic Gel (Dr. Mann Parma, Berlin, Germany) was also put into the eyes so as to prevent eye dryness.

While the body temperature of the mice was maintained at 37° C. on a mouse table (Roland Consult, Wiesbaden, Germany), the responses of retinal nerves to light at intensities of −25, −10, and 0 dB were recoded with the RETIport System (Roland Consult, Wiesbaden, Germany), and the records were analyzed with the RETI-scan System (Roland Consult, Wiesbaden, Germany). The same electroretinography was performed on non-transplanted C57BL6 mice for positive control and on non-transplanted C3H/Prkdc mice (rd/SCID) for negative control.

After the retinoelectrography, the mice were subjected to euthanasia by CO₂ injection and cervical dislocation. The eyeballs were excised, followed by immunofluorescent staining. In detail, the eyeballs excised immediately after euthanasia were fixed overnight at 4° C. with 4% paraformaldehyde, cryoprotected in 10% and 30% sucrose (Sigma-Aldrich), embedded in OCT (Tissue-Tek, Sakura, Tokyo, Japan), and immediately stored at −80° C. Four weeks after the transplantation, some of the mice were subjected to euthanasia by CO₂ injection and cervical dislocation without electroretinography before the excision of the eyeballs. Then, immunofluorescent staining was performed on the eyeballs as described above.

Recovered signals in the electroretinograms reflect the effects and efficacies of the cells transplanted into injured retinas. Particularly, the existence of rhodopsin within the retina of the photoreceptor cell-transplanted mice is closely associated with the amplitude of b-waves. In the electroretinography, non-transplanted rd/SCID mice of the same age (7-9 weeks old) did not respond to light stimuli (FIG. 18A). In contrast, photoreceptor cell-transplanted rd/SCID mice gave definite responses to light stimuli (FIG. 18B). The ERG b-wave from the eyes of the photoreceptor cell-transplanted rd/SCID mice showed characteristic wave forms (FIG. 18B) with an amplitude of 48.4(±3.4) μV (sample size=13) (FIG. 19). Nowhere were characteristic wave forms found in the ERG of the non-transplanted group, which showed a b-wave amplitude of 10.3 (±2.5) μV (sample size=17) different from that of the transplanted group with statistical significance (p<0.0001) (Table 6, FIG. 19).

TABLE 6 Electroretinography of Mice with Retinal Degeneration After Transplantation with hESC-Derived Photoreceptor Cells Non-Transplanted Group Transplanted Group (Mean ± S.E.M.) (Mean ± S.E.M.) (Sample Size = 17) (Sample Size = 13) b-Wave 10.3(±2.5) 48.4(±13.4) Amplitude (μV)

<8-3> Immunochemistry and Evaluation of Transplanted Cells

The cryoprotected segments were sectioned at a thickness of 16 μm and fixed onto glass slides (Muto Pure Chemicals Co., Tokyo, Japan), followed by immunochemical staining with antibodies to human mitochondria and photoreceptor cells (no. of mouse models=13).

When the photoreceptor cells were transplanted into the retina of the mice suffering from retinal degeneration, a new 4- or 5-fold photoreceptor cell layer was formed, comprising the outer nuclear layer consisting of cells characterized by rhodopsin and recoverin (FIG. 20). There was a remarkable difference in the positive rate of rhodopsin and recoverin between photoreceptor cell-transplanted group and non-transplanted cells. In the non-transplanted group, rhodopsin was detected in only two of a total of 199 cells per observation microscopic field (positive rate: 1.0%). In contrast, 88 of a total of 215 cells per microscopic field were rhodopsin positive in the transplanted group, with an outer nuclear layer consisting of a 4- or 5-fold rhodopsin-positive cell layer formed therein (positive rate: 40.8%) (p<0.0001) (Table 7 and FIG. 21).

As a result, the transplanted rod photoreceptor cells were found to occupy approximately 40% of the total area of the retinal sections. Rhodopsin discs, which are embedded in the membrane of the outer segment of photoreceptor cells in the normal retina, were also found in the photoreceptor cells of the transplanted mice (FIG. 20). Primarily structured to convert light energy into electric energy, rhodopsin discs are responsible for the first events in the perception of light. The engrafted/differentiated photoreceptors of the transplanted mice, particularly, the rhodopsin discs of the outer segment were thought to include responses to light stimuli in the electroretinography, amplifying the amplitude of the b-wave.

Recoverin expression was observed in 40 of the total of 168 cells per microscopic field in the non-transplanted group (positive rate: 23.8%), but in 120 of the total 292 cells per microscopic field in the transplanted group (positive rate: 41.0%) (p<0.0001) (Table 7 and FIG. 21). In the non-transplanted group, recoverin means bipolar cells in the inner nuclear layer and cone photoreceptor cells in the outer nuclear layer. In the transplanted group, a 4- or 5-fold recoverin-positive cell layer was formed in the outer nuclear layer as well as in the inner nuclear layer. From these observations, it is understood that when transplanted into an injured retina, hESC-derived photoreceptor cells can graft at proper loci onto the retina and effectively reconstruct the outer nuclear layer which has been lost due to degeneration.

TABLE 7 Evaluation of hESC-Derived Photoreceptor Cells Transplanted into Mice with Retinal Degeneration Non-transplanted Group Transplanted Group No. of Total Positive No. of Total Positive Positive Cell Rate Positive Cell Rate Marker Cells Count (%) Cells Count (%) rhodopsin  2 199 1   88 215 40.8 recoverin 40 168 23.8 120 292 41.0

In addition, the engrafted cells were observed to express synaptophysin, suggesting that they constructed, in synaptic interaction with other intraretinal cells, retinal neurons and light circuits (FIG. 22). That is, the expression of synaptophysin indicates that the photoreceptor cells in the newly formed outer nuclear layer are using synapses to interact with other intraretinal cells to function as a circuit in response to light.

Taken together, the data obtained above implies that when transplanted, the photoreceptor cells differentiated from hESCs according to the present invention can participate in the construction of retinal circuits, exert their own functions in the transplanted subject, and open up the new possibility of giving vision to patients suffering from blindness that is attributed to the loss of photoreceptor cells.

Example 9 Effect of Wnt Signaling Pathway Activator on Proliferation of Retinal Cells

In order to examine the role of a Wnt signaling pathway in differentiation from retinal progenitor cells into retinal cells, various combinations of three differentiation factors Wnt3a (W), Shh (S) and retinoic acid (R) were used (FIG. 18 and Table 8). The differentiation factors were added to media according to culture step and time. Wnt3a was added at a concentration of 50 ng/ml on culture day 13, Shh at a concentration of 250 ng/ml on culture day 18, and retinoic acid at a concentration of 500 nM on culture day 21. In each medium, hESCs were cultured for a total of 29 days before cell counting.

When undifferentiated hESCs were seed at a density of 1.3 (±0.1)×10⁶ cell per well into 6-well cell culture plates, 303±12 floating aggregates were formed. For differentiation, 53 of the floating aggregates (corresponding to 1.55×10⁴ undifferentiated hESCs) were inoculated onto each well.

Statistical analysis of total cell populations in each medium was done using the Kruskal-Wallis test, showing that the five media differed in cell population from one to another with statistical significance (p=0.0026). An additional analysis was conducted to examine differences between groups. FDR-adjusted p-values for one error were obtained and are summarized in Table 8, below.

TABLE 8 Effect of Differentiation Factors on Cell Differentiation Cell Count (×10⁶)(Mean ± S.E.M.) 3.98 0.73 2.21 2.74 0.87 (±0.64) (±0.16) (±0.67) (±0.36) (±0.38) Main Wnt3a + − + + − Factors (50 ng/ml) Shh + + + − − (250 ng/ml) RA (500 nM) + + − + −

The highest cell population was measured upon the use of W+/S+/R+ (3.98 (±0.64)×10⁶ cells), followed by 2.74 (±0.36)×10⁶ cells in W+/S−/R+ and 2.21 (±0.67)×10⁶ cells in W+/S+/R−. W−/S−/R− and W−/S+/R+ proliferated the cells at a density of as low as 0.87 (±0.38)×10⁶ and 0.73 (±0.16)×10⁶, respectively. The data indicate that cell proliferation is independent of the presence of S and R, but depends on the presence of W and that a great part of the cell proliferation results from the effect of Wnt3a. When comparing the two media W+/S+/R+ and W−/S+/R+, the cell populations (3.98×10⁶ vs. 0.73×10⁶) differ by five times. This is another evidence that Wnt3a plays a critical role in cell proliferation.

A statistical analysis showed that there was a difference between W+/S+/R+ and W−/S+/R+ (p=0.03967), between W+/S+/R+ and W−/S−/R− (p=0.03967), and between W−/S+/R+ and W+/S−/R+ (p=0.03967), with significance (p<0.05) (FIG. 21). After being cultured four weeks in the (W+/S+/R+) culture medium for 4 weeks, the undifferentiated cells proliferated to 3.98×10⁶ differentiated cells, which was 257-fold higher than the population of the initial cells.

Example 10 Role of Wnt Signaling Pathway Activator in Differentiation into Retinal Cells

To differentiate the retinal progenitor cells into neural retinal progenitor cells, Dkk-1 supply was quit on day 13 after induction and a culture medium containing a combination of noggin, IGF-1, FGF2 and Wnt3a (50 ng/ml) was supplied for 5 days. Wnt3a must be removed from the culture media so as to induce and promote the development of the forebrain and eye field precursors during the early embryogenesis. However, Pax6-positive cells increased in a dose-dependent manner with increase in Wnt3a concentration. Pax6 is expressed upon the development of the retina during embryogenesis, that is, on all proliferating retinal progenitor cells (Marquardt & Gruss. Trends Neurosci., 2002; 25: 32-8) in addition to being essential for the generation of neural retinal progenitor cells. Accordingly, an increased expression level of Pax6 is indicative of highly efficient differentiation into neural retinal progenitor cells. Wnt3a was supplied, with the concomitant removal of the Wnt3a inhibitor, in order to induce the expression of Pax6 at a high rate. The expression levels of retinal cell markers before and after Wnt3a addition are given in Table 9, below.

TABLE 9 Expression Levels of Retinal Cell Markers before and after Wnt3a Addition Mean ± S.E.M. (%, Sample size = 3) Statistical Before Wnt3 a After Wnt3 a addition Significance Marker addition (on Day 13) (on Day 18) (p value) Rax 86.6 (±3.0) 98.2 (±0.9) <0.0001 Pax6 63.9 (±0.9) 89.1 (±2.5) <0.0001 Otx2 76.4 (±2.0) 61.5 (±0.6) <0.0001 Sox2 83.0 (±1.9) 68.1 (±1.1) <0.0001 Chx10 46.3 (±1.0) 64.5 (±1.6) <0.0001 Nestin 65.7 (±2.7) 18.0 (±1.4) <0.0001 Mitf 17.2 (±0.4)  2.7 (±1.0) <0.0001 Crx 12.8 (±1.6) 80.1 (±0.2) <0.0001 Recoverin 35.8 (±0.4) 68.5 (±2.6) <0.0001 Rhodopsin 35.3 (±1.7) 52.9 (±3.4) <0.0001 Peripherin2  5.6 (±0.3) 13.9 (±2.4) <0.0001 Ki67 87.5 (±1.3) 31.5 (±0.5) <0.0001

On the differentiated neural retinal progenitor cells, the positive rates were increased simultaneously from 63.9% to 89.1% for Pax 6 (p<0.0001), from 86.6% to 98.2% for Rax (p<0.0001), and from 46.3% to 64.5% for Chx10 (p<0.0001), indicating that the increase in the positive rate of Pax6 resulted from the proliferation of neural retinal progenitor cells (both Rax+ and Pax6+ positive) (Table 9).

Nestin, a marker characteristic of most CNS neural progenitoc cells, which is known to be not expressed in neural retinal progenitor cells (Yang et al, Mech. Dev. 2000; 94: 287-91), was greatly decreased in positive rate from 65.7% to 18.0% after Wnt3a addition (p<0.0001), from which it is understood that the Wnt signaling pathway restrained the proliferation of neural progenitor cells and promoted the proliferation and differentiation of neural retinal cells.

On the other hand, the proliferative cell marker Ki67 rapidly decreased in positive rate from 87.5% to 31.5% (p<0.0001), indicating that differentiation was more active than proliferation. Immediately after neural retinal progenitor cells leave the cell proliferation cell cycle, photoreceptor cell precursors are generated with the concomitant expression of Crx. Thus, the Wnt signaling pathway induced the neural retinal progenitor cells leaving the cell proliferation cycle to differentiate into Crx-expressing photoreceptor cell precursors (Table 9).

With regard to the signal pathway for inducing differentiation from retinal progenitor cells into Crx-positive photoreceptor cell precursors in vivo, its exact biological changes and molecules have not yet been reported in any prior art, so far (Ikeda et al., Proc. Natl. Acad. Sci. USA, 2005; 102: 11331-6). In the present invention, however, Wnt3a was first found to very strongly induce the expression of the photoreceptor cell precursor marker Crx. That is, the positive rate of Crx showed a drastic increase from 12.8% before Wnt3a addition to 80.1% after Wnt3a addition (p<0.0001) (Table 9). The increased expression level of Crx had on influence the differentiation and generation of photoreceptor cells, directly promoting differentiation into photoreceptor cells. An increased expression level was also found in the universal photoreceptor cell marker recoverin (from 35.8% to 68.5%, p<0.0001), in the rod photoreceptor cell marker rhodopsin (from 35.3% to 52.9%, p<0.0001), and in the photoreceptor cell's outer segment marker peripherin2 (from 5.6% to 13.9%, p<0.0001) (Table 9)

Mitf, an antigen marker of retinal pigmented epithelial progenitor cells, showed a decrease in positive rate from 17.2% to 2.7% (p<0.0001), demonstrating that the Wnt signaling pathway promoted the generation of neural retinal cells and photoreceptor cells, but prevents the formation of retinal pigmented epithelium (Table 9).

Example 11 Role of Inhibitor of Wnt Signaling Pathway Activator in Differentiation into Retinal Cells <11-1> Addition of Wnt Signaling Pathway Antagonist

In order to examine the function of the Wnt signaling pathway activator on differentiation from hESCs into retinal cells, particularly photoreceptor cells, cells were cultured in the presence of a Wnt signaling pathway antagonist. The Wnt/Frizzled signaling pathway inhibitor Dkk-1 (R&D Systems, Minneapolis, Minn., USA) was added at a concentration of 10 ng/ml, 100 ng/ml or 1 μg/ml on day 13, followed by culturing the cells for an additional 16 days. While the antagonist concentrations were maintained uniformly over the culture time period, the Wnt signaling pathway activator was assayed for activity. Dkk-1 was dissolved in 0.1% bovine serum albumin (BSA, Sigma-Aldrich) in PBS before use.

According to a protocol for differentiation into retinal cells, Wnt3a and Dkk-1 were added on Day 13 at the following concentrations: 50 ng/ml Wnt3a (group I), free of both Wnt3a and Dkk-1 (group II), 10 ng/ml Dkk-1 (group III), 100 ng/ml Dkk-1 (group IV), and 1 μ/ml Dkk-1 (group V). In each medium, the cells were cultured to Day 29. In this experiment, none of the other major differentiation factors Shh and RA were used.

<11-2> Immunofluorescent Staning

To examine the effect of the Wnt signaling pathway activator on the cell differentiation of Example 11-1, an immunofluorescent staining analysis was performed as follows. On Day 29, the cells which had been cultured on poly-D-lysine/laminin-coated, 8-well slides (BD Biosciences) under the conditions of Wnt3a and Dkk-1 (groups I-V) were fixed with 4% paraformaldehyde (Sigma-Aldrich), after which non-specific reactions were blocked with PBS containing 3% BSA (Jackson Immunoresearch Laboratory, Bar Harbor, Me., USA) and 0.25% Triton X-100 (Sigma-Aldrich).

After being blocked for 90 min, the slides in each differentiation step were incubated overnight at 4° C. with the following antibodies specific for cells of each differentiation step: rabbit-Blue-opsin (1:500, Chemicon), sheep-Chx10 (1:100, Chemicon), rabbit-Crx (1:200, Santa Cruz Biotechnology, Santa Cruz, Calif., USA), rabbit-GFAP (1:200, Invitrogen), mouse-human specific mitochondria (1:50, Chemicon), rabbit-human specific mitochondria (1:200, Chemicon), mouse-Islet1 (1:10, Developmental Studies Hydroma Bank, DSHB; Iowa City, Iowa, USA), mouse-Ki67 (1:100, Vector Laboratories, Peterborough, England), mouse-Mitf (1:1,000, abcam, Cambridge, Mass., USA), mouse-nestin (1:250, BD Sciences), rabbit-neurofilament-200 (1:1,000, Sigma-Aldrich), rabbit-Otx2 (1:100, abcam), rabbit-Pax2 (1:250, abcam), mouse-Pax6 (1:2, DSHB), mouse-peripherin2 (1:500, GenScript, Piscataway, N.J., USA), rabbit-Rax (1:250, abcam), rabbit-recoverin (1:1,000, Chemicon), retinal pigment epithelium 65 (RPE65, 1:100, Chemicon), rabbit-rhodopsin (1:500, Sigma-Aldrich), mouse-rhodopsin (1:500, Ret-P1, Lab Vision, Fremont, Calif., USA), mouse-rhodopsin (1:2,000, Ret-P1, Sigma-Aldrich), mouse-rom1 (1:50, ABR-Affinity Bioreagents, Golden, Colo., USA), rabbit-PDE6 beta (1:100, abcam), rabbit-phosducin (1:500, Santa Cruz Biotechnology), mouse-PKC-alpha (1:500, Sigma-Aldrich), mouse-Prox1 (1:2,000, Chemicon), mouse-Sox2 (1:250, R&D Systems), rabbit-synaptophysin (1:2,000, Santa Cruz Biotechnology), and rabbit-ZO-1 (1:100, Zymed-Invitrogen).

Before use, these antibodies were diluted in a PBS solution containing 1% BSA and 0.25% Triton X-100. The cells were washed three times for 5 min with PBS and incubated at room temperature for 2 hrs with species-specific secondary antibody conjugated with Cy3 (1:800, Jackson Immunoresearch Laboratory) or Alexa488 (1:500, Invitrogen). A standard material suitable for the primary and the secondary antibody was used to examine non-specific staining or interaction between the antibodies. Afterwards, the cells were washed three times for 5 min with PBS, counterstained with DAPI (4,6-diamidino-2 phenylindole) and mounted in Vectashield (Vector Laboratories), followed by visualization under an epifluorescence microscope (Nikon Eclipse, E800, Tokyo, Japan) and a confocal microscope (Leica, Leica Microsystems Inc, Bannockburn, Ill., USA or Zeiss LSM510, Carl Zeiss, Inc, Thornwood, N.Y., USA).

500 cells were counted from 20 microscopic fields randomly selected at 400× magnification and evaluated for positive responses to each antibody. Positive responses to antibodies were determined after at least three evaluations. After the data was corrected for cluster effects, statistical analysis of the data was done using the GEE (Generalized Estimating Equations) model of SAS version 9.1 so as to investigate changes in positive rate with concentrations of Wnt3a and Dkk-1. All data were represented as mean±standard error of the mean (S.E.M) with a statistical significance of p<0.05.

As a result, significant differences were detected among the photoreceptor cell marker. The photoreceptor cell markers Crx (p=0.0247), recoverin (p=0.0113), rhodopsin (p=0.0166) and peripherin2 (p=0.0166) remarkably decreased in expression level with increasing of Dkk-1 concentration. Statistical significances were found between all groups except for group I vs. group II and group IV vs. group V in Crx and group I vs. group II in recoverin (significance between groups: *: p<0.0001; **: p=0.0381) (Tables 10 and 11 and FIG. 24). These data showed that the differentiated photoreceptor cells resulted from the activation of the Wnt signaling pathway and that the inhibition of the Wnt signaling pathway led to almost no generation of photoreceptor cells.

The rhodopsin which was expressed at a positive rate of 11.9% in spite of 1 μg/ml Dkk-1 was thought to be attributed to the stabilization of beta-catenin by a Wnt independent mechanism or the activation of non-canonical pathway by Wnt, Wnt5 and Wnt11. Also, the expression level of the retinal progenitor cell marker Pax6 significantly differed from one group to another over the overall 5 groups (p=0.0275) and was decreased with an increase in, Dkk-1 concentration (all p values between groups: p<0.0001) (FIG. 24, Tables 9 and 10).

TABLE 10 Positive Rates of Photoreceptor Cell Markers According to Concentrations of Wnt3a and its inhibitor Dkk-1 Mean ± S.E.M. (%, Sample size = 3) Wnt3a, 50 Wnt3a (−), Dkk-1, 100 ng/ml Dkk-1 (−) Dkk-1, 10 ng/ml ng/ml Dkk-1, 1 μg/ml, Marker (Group I) (Group II) (Group III) (Group IV) (Group V) Pax6 82.1 (±4.0) 64.0 (±1.0) 63.9 (±0.9) 51.8 (±1.0) 42.1 (±2.1) Crx 32.5 (±0.3) 31.8 (±3.8) 54.3 (±3.1)  7.4 (±1.1)  7.5 (±0.9) Recoverin 72.3 (±0.8) 71.1 (±0.8) 55.1 (±0.6) 41.5 (±1.3) 15.2 (±4.0) Rhodopsin 73.8 (±0.9) 52.7 (±1.0) 49.1 (±2.0) 28.9 (±0.5) 11.9 (±3.0) Peripherin2 21.5 (±0.3) 16.4 (±0.8) 27.0 (±1.0)  2.1 (±1.0)  0.1 (±0.1)

TABLE 11 Statistical Significances of Positive Rates of Photoreceptor Cell Markers According to Concentrations of Wnt3a and its Inhibitor Dkk-1 Statistical Significance (p value) Group I vs. Group II vs. Group III vs. Group IV vs. Marker Group II Group III Group IV Group V Pax6 <0.0001 0.9036 <0.0001 <0.0001 Crx 0.8147 <0.0001 <0.0001 1.0000 Recoverin 0.1864 <0.0001 <0.0001 <0.0001 Rhodopsin <0.0001 0.0381 <0.0001 <0.0001 Peripherin2 <0.0001 <0.0001 <0.0001 <0.0001 *Statistical significance: p < 0.05 *Group I: Wnt3a, 50 ng/ml; Group II: Wnt3a (−), Dkk-1 (−); Group III: Dkk-1, 10 ng/ml; Group IV: Dkk-1, 100 ng/ml; Group V: Dkk-1, 1 μg/ml.

Example 12 Positive Rates of Retinal Cell Markers Upon Use of Substitutes for Wnt3a and Shh

Differentiation from retinal progenitor cells into neural retinal progenitor cells, from neural retinal progenitor cells into photoreceptor cell precursors, and from photoreceptor cells into photoreceptor cells was induced in the same manner as in Examples 4 to 6, with the exception that the media used in Examples 4 to 6 contained 50 ng/ml recombinant Wnt1 (PeproTech, Rocky Hill, N.J., USA), 50 or 100 ng/ml recombinant Wnt5A (R&D Systems), 50 or 100 ng/ml recombinant Wnt11 (R&D Systems), 50 ng/ml recombinant Norrin (R&D Systems), 2.5 mM and 5 mM LiCl (Sigma-Aldrich), 2 μM BIO (Sigma-Aldrich), or 30 μM SB415286 (Sigma-Aldrich), instead of the recombinant Wnt3a, and with the further exception that the media used in Examples 5 and 6 contained 1 μM purmorphamine (Stemgent, Cambridge, Mass., USA) instead of the recombinant Shh.

The cells differentiated with various substitutes for Wnt3a and Shh were subjected to immunochemical staining in the same manner as in Example 7-1 and assayed for positive rates of retinal cell markers, and the results are summarized in Tables 12 and 13, below.

TABLE 12 Positive Rates of Retinal Cell Markers upon Use of Wnt3a Substitutes Mean ± S.E.M (%, Sample size = 3) Substitute recoverin rhodopsin Rom peripherin2 Crx Ki67 Pax6 b-opsin Wnt1 70.9(±1.8) 66.2(±5.4) 55.9(±2.9)  3.8(±2.0) 60.3(±1.3) 23.8(±1.2) ND 61.6(±6.8) (50 ng/ml) Wnt5A 61.0(±3.2) 59.5(±2.0) 58.8(±2.7) 20.7(±0.8) 67.3(±1.9) 41.3(±3.9) ND 55.0(±4.4) (50 ng/mL) Wnt5A 61.6(±2.5) 56.1(±1.8) ND 14.2(±2.8) ND ND ND ND (100 ng/ml) Wnt11 65.3(±2.7) 51.8(±7.7) 50.2(±7.4)  0.5(±0.3) 65.9(±5.8) 40.1(±5.2) ND 30.6(±6.2) (50 ng/ml) Wnt11 61.8(±4.1) 59.1(±5.4) ND 10.7(±2.9) ND ND ND ND (100 ng/ml) Norrin 85.0(±7.2) 82.9(±7.1) 69.5(±2.7) 24.9(±4.5) 39.3(±0.8) 42.2(±6.8) ND 71.1(±0.6) (50 ng/ml) BIO (2 uM) 77.1(±0.8) 71.5(±1.2) 60.3(±0.9) 33.9(±0.1) 65.8(±1.0) 17.2(±0.6) 92.8(±1.0) 77.5(±1.8) SB415286 70.0(±0.6) 69.3(±0.5) 59.5(±0.7)  9.1(±0.6) 53.7(±2.7) 13.3(±1.2) 89.1(±0.6) 53.7(±2.7) (30 uM) LiCl 64.1(±2.9) 56.2(±1.7) ND ND ND ND ND ND (2.5 mM) LiCl 78.9(±4.4) 68.5(±2.2) ND  1.5(±0.9)  5.0(±1.7) 33.3(±2.2) ND ND (5 mM) *ND: Non-determined.

TABLE 13 Positive Rates of Retinal Cell Markers upon Use of Shh Substitutes Mean ± S.E.M (%, Sample size = 3) Substitute recoverin rhodopsin peripherin2 Crx Ki67 purmorpha- 76.5(±0.9) 73.5(±1.2) 41.5(±1.2) 71.6(±0.8) 24.3(±0.5) mine (1 uM)

As is apparent from the data of Tables 12 and 13, the Wnt3a substitutes, Wnt1, Wnt5a, Wnt11, norrin, LiCl, BIO and SB415286 and the Shh substitute purmorphamine allowed the retinal cell markers to be expressed at similar positive rates to those when Wnt3a and Shh were used. 

1. A method for inducing proliferation of retinal cells, comprising the step of culturing the retinal cells in a medium containing a Wnt signaling pathway activator.
 2. The method as set forth in claim 1, wherein the retinal cells are photoreceptor cells.
 3. The method as set forth in claim 1, wherein the retinal cells are human-derived cells.
 4. The method as set forth in claim 1, wherein the Wnt signaling pathway activator is selected from the group consisting of Wnt1, Wnt2, Wnt2b, Wnt3, Wnt3a, Wnt4, Wnt5a, Wnt5b, Wnt6, Wnt7a, Wnt7b, Wnt8a, Wnt8b, Wnt9a, Wnt9b, Wnt10a, Wnt10b, Wnt11, Wnt16b; β-catenin expression level-increasing substances; Axin inhibitors; APC (adenomatous polyposis coli) inhibitors; norrin; R-spondin 2; and a combination thereof.
 5. The method as set forth in claim 4, the Wnt signaling pathway activator is selected from the group consisting of Wnt1, Wnt3a, Wnt5a, Wnt11, norrin and a combination thereof.
 6. The method as set forth in claim 1, wherein the Wnt signaling pathway activator is a GSK3 (glycogen synthase kinase 3) inhibitor.
 7. The method as set forth in claim 6, wherein the GSK3 inhibitor is selected from the group consisting of lithium (Li), LiCl, bivalent zinc (bivalent Zn), BIO (6-bromoindirubin-3′-oxime), SB216763, SB415286, CHIR99021, QS11 hydrate, TWS119, kenpaullone, alsterpaullone, indirubin-3′-oxime, TDZD-8, Ro 31-8220 methanesulfonate salt and a combination thereof.
 8. The method as set forth in claim 7, wherein the GSK3 inhibitor is selected from the group consisting of LiCl, BIO (6-bromoindirubin-3′-oxime), SB415286 and a combination thereof.
 9. The method as set forth in claim 4, wherein the concentration of the Wnt signaling pathway activator used in the medium is ranging from 0.01 to 500 ng/ml.
 10. The method as set forth in claim 9, the concentration of the Wnt signaling pathway activator used in the medium is ranging from 1 to 100 ng/ml.
 11. The method as set forth in claim 7, wherein the concentration of the GSK inhibitor except for LiCl, BIO and SB415286 used in the medium is ranging from 0.01 to 500 ng/ml.
 12. The method as set forth in claim 11, wherein the concentration of the GSK inhibitor except for LiCl, BIO and SB415286 used in the medium is ranging from 1 to 100 ng/ml.
 13. The method as set forth in claim 7, wherein the concentration of LiCl used in the medium is ranging from 0.1 to 50 mM; that of BIO is ranging from 0.1 to 50 μM; and that of SB415286 is ranging from 0.1 to 500 μM.
 14. The method as set forth in claim 13, wherein the concentration of LiCl used in the medium is ranging from 1 to 10 mM; that of BIO is ranging from 0.5 to 5 μM; and that of SB415286 is ranging from 5 to 50 μM.
 15. A method for inducing differentiation from retinal progenitor cells into retinal cells, comprising the step of culturing the retinal progenitor cells in a medium containing a Wnt signaling pathway activator.
 16. The method as set forth in claim 15, wherein the retinal cells are photoreceptor cells.
 17. The method as set forth in claim 15, which comprises: (a) culturing the retinal progenitor cells in a medium containing a Wnt signaling pathway activator to differentiate them into neural retinal progenitor cells; (b) culturing the neural retinal progenitor cells of step (a) in a medium containing a Wnt signaling pathway activator to differentiate them into photoreceptor cell precursors; and (c) culturing the photoreceptor cell precursors of step (b) in a medium containing a Wnt signaling pathway activator to differentiate them into photoreceptor cells or retinal cells including photoreceptor cells.
 18. The method as set forth in claim 15, wherein the Wnt signaling pathway activator is selected from the group consisting of Wnt1, Wnt2, Wnt2b, Wnt3, Wnt3a, Wnt4, Wnt5a, Wnt5b, Wnt6, Wnt7a, Wnt7b, Wnt8a, Wnt8b, Wnt9a, Wnt9b, Wnt10a, Wnt10b, Wnt11, Wnt16b; β-catenin expression level-increasing substances; Axin inhibitors; APC (adenomatous polyposis coli) inhibitors; norrin; R-spondin 2; and a combination thereof.
 19. The method as set forth in claim 18, wherein the Wnt signaling pathway activator is selected from the group consisting of Wnt1, Wnt3a, Wnt5a, Wnt11, norrin and a combination thereof.
 20. The method as set forth in claim 15, wherein the Wnt signaling pathway activator is a GSK3 (glycogen synthase kinase 3) inhibitor.
 21. The method as set forth in claim 20, wherein the GSK3 inhibitor is selected from the group consisting of lithium (Li), LiCl, bivalent zinc (bivalent Zn), BIO (6-bromoindirubin-3′-oxime), SB216763, SB415286, CHIR99021, QS11 hydrate, TWS119, kenpaullone, alsterpaullone, indirubin-3′-oxime, TDZD-8, Ro 31-8220 methanesulfonate salt and a combination thereof.
 22. The method as set forth in claim 21, wherein the GSK3 inhibitor is selected from the group consisting of LiCl, BIO (6-bromoindirubin-3′-oxime), SB415286 and a combination thereof.
 23. The method as set forth in claim 15, wherein the concentration of the Wnt signaling pathway activator used in the medium is ranging from 0.01 to 500 ng/ml.
 24. The method as set forth in claim 23, the concentration of the Wnt signaling pathway activator used in the medium is ranging from 1 to 100 ng/ml.
 25. The method as set forth in claim 21, wherein the concentration of the GSK inhibitor except for LiCl, BIO and SB415286 used in the medium is ranging from 0.01 to 500 ng/ml.
 26. The method as set forth in claim 25, wherein the concentration of the GSK inhibitor except for LiCl, BIO and SB415286 used in the medium is ranging from 1 to 100 ng/ml.
 27. The method as set forth in claim 21, wherein the concentration of LiCl used in the medium is ranging from 0.1 to 50 mM; that of BIO is ranging from 0.1 to 50 μM; and that of SB415286 is ranging from 0.1 to 500 μM.
 28. The method as set forth in claim 27, wherein the concentration of LiCl used in the medium is ranging from 1 to 10 mM; that of BIO is ranging from 0.5 to 5 μM; and that of SB415286 is ranging from 5 to 50 μM. 